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

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

Since one error message will occlude others I had to split the files as I did in the latest commit.
I have added one for the `_Alignof` case checking for the errors I was getting in godbolt while also checking that no `undefined-internal` error occurred.
Lastly, I added the one illegal use case of `_Generic` from @AaronBallman 's suggestions.
I think this should cover everything.
What do you think?

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