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

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

Ah I see. Thank you for the suggestions!
I also tried the same for `_Alignof` but was put off by the errors generated by the `-pedantic-errors` flag.
But I guess I can just check that I don't get the `undefined-internal` error.

For the other two I had something similar but that still misses this point:
— part of the expression in a generic association that is not the result expression of its generic
selection
But that might also be because I don't understand this point well enough. I don't see which expression is there that is not the controlling expression and not a result expression.

Also, I was thinking that it would be best to group all tests into one file since they all test just one feature.
What do you think of that?

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