[clang] [clang][test] Use sizeof(wchar_t) instead of __WCHAR_WIDTH__ (PR #218707)

[Vladimir Vereschaka via cfe-commits]

vvereschaka wrote:

@tbaederr,

looks like these changes cause the `Clang-Unit::AllClangUnitTests.exe/Pointer/Strings` test failure on the Windows host
https://lab.llvm.org/buildbot/#/builders/2/builds/60133

``` 
******************** TEST 'Clang-Unit :: ./AllClangUnitTests.exe/97/106' FAILED ********************
Script(shard):
--
GTEST_OUTPUT=json:C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\tools\clang\unittests\.\AllClangUnitTests.exe-Clang-Unit-11896-97-106.json GTEST_SHUFFLE=0 GTEST_TOTAL_SHARDS=106 GTEST_SHARD_INDEX=97 C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\tools\clang\unittests\.\AllClangUnitTests.exe
--
Script:
--
C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\build\tools\clang\unittests\.\AllClangUnitTests.exe --gtest_filter=Pointer.Strings
--
C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\llvm-project\clang\unittests\AST\ByteCode\Pointer.cpp(318): error: Expected equality of these values:
  Pointee.elemSize()
    Which is: 4
  sizeof(wchar_t)
    Which is: 2
C:\buildbot\as-builder-3\llvm-clang-x86_64-win-fast\llvm-project\clang\unittests\AST\ByteCode\Pointer.cpp:318
Expected equality of these values:
  Pointee.elemSize()
    Which is: 4
  sizeof(wchar_t)
    Which is: 2
********************
```

https://github.com/llvm/llvm-project/pull/218707