feat(core): use literal for stack.partition (under feature flag) (#21420

)

closes #4092

When a Stack's region is a string literal, we can return a string
literal from Stack.partition, resulting in easier to read generated
templates.

The feature flag to enable this behavior is `@aws-cdk/core:enablePartitionLiterals`.

----

### All Submissions:

* [X] Have you followed the guidelines in our [Contributing guide?](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md)

### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [x] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [x] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

packages/@aws-cdk/aws-codepipeline-actions/test/pipeline-cfn-cross-region.integ.snapshot/aws-cdk-codepipeline-cloudformation-cross-region.assets.json

@@ -1,5 +1,5 @@
 {
-  "version": "17.0.0",
+  "version": "20.0.0",
   "files": {
     "64e4857ead2a6d40a579c8325f31ac188170c3658ab06c2020de84e581a6069d": {
       "source": {

packages/@aws-cdk/aws-codepipeline-actions/test/pipeline-cfn-cross-region.integ.snapshot/aws-cdk-codepipeline-cloudformation-cross-region.template.json

@@ -209,11 +209,7 @@
          "Fn::Join": [
           "",
           [
-           "arn:",
-           {
-            "Ref": "AWS::Partition"
-           },
-           ":iam::",
+           "arn:aws:iam::",
            {
             "Ref": "AWS::AccountId"
            },
@@ -321,11 +317,7 @@
          "Fn::Join": [
           "",
           [
-           "arn:",
-           {
-            "Ref": "AWS::Partition"
-           },
-           ":iam::",
+           "arn:aws:iam::",
            {
             "Ref": "AWS::AccountId"
            },
@@ -400,11 +392,7 @@
         "Fn::Join": [
          "",
          [
-          "arn:",
-          {
-           "Ref": "AWS::Partition"
-          },
-          ":cloudformation:us-west-2:",
+          "arn:aws:cloudformation:us-west-2:",
           {
            "Ref": "AWS::AccountId"
           },

packages/@aws-cdk/aws-codepipeline-actions/test/pipeline-cfn-cross-region.integ.snapshot/cdk.out

@@ -1 +1 @@
-{"version":"17.0.0"}
+{"version":"20.0.0"}

packages/@aws-cdk/aws-codepipeline-actions/test/pipeline-cfn-cross-region.integ.snapshot/manifest.json

@@ -1,5 +1,5 @@
 {
-  "version": "17.0.0",
+  "version": "20.0.0",
   "artifacts": {
     "Tree": {
       "type": "cdk:tree",

packages/@aws-cdk/aws-codepipeline-actions/test/pipeline-cfn-cross-region.integ.snapshot/tree.json

@@ -9,7 +9,7 @@
         "path": "Tree",
         "constructInfo": {
           "fqn": "constructs.Construct",
-          "version": "10.0.9"
+          "version": "10.1.65"
         }
       },
       "aws-cdk-codepipeline-cloudformation-cross-region": {
@@ -300,11 +300,7 @@
                                           "Fn::Join": [
                                             "",
                                             [
-                                              "arn:",
-                                              {
-                                                "Ref": "AWS::Partition"
-                                              },
-                                              ":iam::",
+                                              "arn:aws:iam::",
                                               {
                                                 "Ref": "AWS::AccountId"
                                               },
@@ -431,13 +427,13 @@
                     },
                     "constructInfo": {
                       "fqn": "constructs.Construct",
-                      "version": "10.0.9"
+                      "version": "10.1.65"
                     }
                   }
                 },
                 "constructInfo": {
                   "fqn": "constructs.Construct",
-                  "version": "10.0.9"
+                  "version": "10.1.65"
                 }
               },
               "CFN": {
@@ -468,11 +464,7 @@
                                           "Fn::Join": [
                                             "",
                                             [
-                                              "arn:",
-                                              {
-                                                "Ref": "AWS::Partition"
-                                              },
-                                              ":iam::",
+                                              "arn:aws:iam::",
                                               {
                                                 "Ref": "AWS::AccountId"
                                               },
@@ -497,7 +489,7 @@
                             "path": "aws-cdk-codepipeline-cloudformation-cross-region/MyPipeline/CFN/CFN_Deploy/CodePipelineActionRole/8389e75f-0810-4838-bf64-d6f85a95cf83",
                             "constructInfo": {
                               "fqn": "constructs.Construct",
-                              "version": "10.0.9"
+                              "version": "10.1.65"
                             }
                           },
                           "DefaultPolicy": {
@@ -567,11 +559,7 @@
                                             "Fn::Join": [
                                               "",
                                               [
-                                                "arn:",
-                                                {
-                                                  "Ref": "AWS::Partition"
-                                                },
-                                                ":cloudformation:us-west-2:",
+                                                "arn:aws:cloudformation:us-west-2:",
                                                 {
                                                   "Ref": "AWS::AccountId"
                                                 },
@@ -710,13 +698,13 @@
                     },
                     "constructInfo": {
                       "fqn": "constructs.Construct",
-                      "version": "10.0.9"
+                      "version": "10.1.65"
                     }
                   }
                 },
                 "constructInfo": {
                   "fqn": "constructs.Construct",
-                  "version": "10.0.9"
+                  "version": "10.1.65"
                 }
               }
             },