[clang-tools-extra] [clang-tidy] Fix for cppcoreguidelines-pro-type-union-access if memLoc is invalid (PR #104540)

[Nicolas van Kempen via cfe-commits]

https://github.com/nicovank approved this pull request.

LGTM! Minus alphabetical ordering in release notes. I'll merge this tomorrow to give time to others just in case.

FWIW this issue actually triggers a crash/assertion failure when those are enabled:
```
clang-tidy: clang-tools-extra/clang-tidy/ClangTidyDiagnosticConsumer.cpp:178: DiagnosticBuilder clang::tidy::ClangTidyContext::diag(StringRef, SourceLocation, StringRef, DiagnosticIDs::Level): Assertion `Loc.isValid()' failed.
[...]
```

https://github.com/llvm/llvm-project/pull/104540