[clang] [Modules] Fix the inconsistency of which `Decl` should be serialized for an identifier. (PR #135887)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `flang-x86_64-windows` running on `minipc-ryzen-win` while building `clang` at step 8 "test-build-unified-tree-check-flang-rt".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/166/builds/1158

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 8 (test-build-unified-tree-check-flang-rt) failure: test (failure)
******************** TEST 'flang-rt-OldUnit :: Runtime/RuntimeTests.exe' FAILED ********************
[==========] Running 225 tests from 54 test suites.

[----------] Global test environment set-up.

[----------] 4 tests from AllocatableTest

[ RUN      ] AllocatableTest.MoveAlloc

[       OK ] AllocatableTest.MoveAlloc (0 ms)

[ RUN      ] AllocatableTest.AllocateFromScalarSource

[       OK ] AllocatableTest.AllocateFromScalarSource (0 ms)

[ RUN      ] AllocatableTest.AllocateSourceZeroSize

[       OK ] AllocatableTest.AllocateSourceZeroSize (0 ms)

[ RUN      ] AllocatableTest.DoubleAllocation

[       OK ] AllocatableTest.DoubleAllocation (0 ms)

[----------] 4 tests from AllocatableTest (0 ms total)



[----------] 3 tests from ArrayConstructor

[ RUN      ] ArrayConstructor.Basic

[       OK ] ArrayConstructor.Basic (0 ms)

[ RUN      ] ArrayConstructor.Character

[       OK ] ArrayConstructor.Character (0 ms)

[ RUN      ] ArrayConstructor.CharacterRuntimeCheck

[       OK ] ArrayConstructor.CharacterRuntimeCheck (72 ms)

[----------] 3 tests from ArrayConstructor (72 ms total)



[----------] 1 test from BufferTests

[ RUN      ] BufferTests.TestFrameBufferReadAndWrite

[       OK ] BufferTests.TestFrameBufferReadAndWrite (0 ms)
...

```

</details>

https://github.com/llvm/llvm-project/pull/135887