[clang] [clang] Fix isInStdNamespace for Decl flagged extern c++ (PR #81776)

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https://github.com/cor3ntin commented:

Looks good.
Can you add an entry to clang/docs/ReleseNotes.rst? 
There is an ast matchers section.

Thanks

https://github.com/llvm/llvm-project/pull/81776