No tfcompat check if ReferenceOutputAssembly=false

It is confusing to explicitly specify that you _don't_ want the output
of another project referenced in this project and then be told that the
output is incompatible.

This commit listens to the preexisting ProjectReference metadatum
ReferenceOutputAssembly and avoids the compatibility/best-match checks
on ProjectReferences that avoid the dependency.

Fixes dotnet#2661 (and dotnet/sdk#939).

src/Tasks/Microsoft.Common.CurrentVersion.targets

@@ -1543,9 +1543,15 @@ Copyright (C) Microsoft Corporation. All rights reserved.
 
       We give this treatment to .vcxproj by default since no .vcxproj can target more
       than one framework.
+
+      Likewise if the dependency is for build ordering instead of an assembly reference
+      (ReferenceOutputAssembly=false), skip the checks since we can't know what TF
+      the output would need to be compatible with.
    -->
    <ItemGroup>
-      <_MSBuildProjectReferenceExistent Condition="'%(_MSBuildProjectReferenceExistent.SkipGetTargetFrameworkProperties)' == '' and '%(Extension)' == '.vcxproj'">
+      <_MSBuildProjectReferenceExistent
+        Condition="'%(_MSBuildProjectReferenceExistent.SkipGetTargetFrameworkProperties)' == '' and
+                   ('%(Extension)' == '.vcxproj' or '%(ReferenceOutputAssembly)' == 'false'">
         <SkipGetTargetFrameworkProperties>true</SkipGetTargetFrameworkProperties>
       </_MSBuildProjectReferenceExistent>
    </ItemGroup>