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

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

Ok, that's done with the FIXME comment for now.
Maybe it is only because this is my first time contributing and I am not too familiar with the codebase but I couldn't see a trivial fix for the issue.
For now I think it's best to land these changes first.
I would just add an issue about it.

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