Checker: fix exception on wrong update syntax in anon records

[auduchinok]

Fixes exceptions introduced in #15214 when wrong syntax is used to update an anonymous record:

let f (r: {| A: int |}) =
    { r with A = 1 }

[KevinRansom]

nice

[vzarytovskii]

/backport to release/dev17.7

[github-actions]

Started backporting to release/dev17.7: https://github.com/dotnet/fsharp/actions/runs/5654806378

[vzarytovskii]

Backport is needed to resolve merge issues in 17.7 -> main, it won't go into QB.

[github-actions]

@vzarytovskii backporting to release/dev17.7 failed, the patch most likely resulted in conflicts:

$ git am --3way --ignore-whitespace --keep-non-patch changes.patch

Applying: Revert "Fix: 14814 --- Inaccurate error in anonymous record copy-and-update (#15625)"
Applying: Checker: fix exception in wrong anon record update
Using index info to reconstruct a base tree...
M	src/Compiler/Checking/CheckExpressions.fs
M	tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/AnonymousRecords.fs
Falling back to patching base and 3-way merge...
Auto-merging tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/AnonymousRecords.fs
CONFLICT (content): Merge conflict in tests/FSharp.Compiler.ComponentTests/Conformance/Types/RecordTypes/AnonymousRecords.fs
Auto-merging src/Compiler/Checking/CheckExpressions.fs
error: Failed to merge in the changes.
hint: Use 'git am --show-current-patch=diff' to see the failed patch
Patch failed at 0002 Checker: fix exception in wrong anon record update
When you have resolved this problem, run "git am --continue".
If you prefer to skip this patch, run "git am --skip" instead.
To restore the original branch and stop patching, run "git am --abort".
Error: The process '/usr/bin/git' failed with exit code 128

Please backport manually!