[clang] fix bug that undefined internal is a warning only for -pedantic-errors (PR #98016)

[via cfe-commits]

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

Thanks for the fix! 

The changes LGTM, but you should also add a release note about this (in `clang/docs/ReleaseNotes.rst`, probably in the ‘Improvements to Clang’s Diagnostics’ section).

https://github.com/llvm/llvm-project/pull/98016