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

[Constantin Kronbichler via cfe-commits]

ccrownhill wrote:

I added three more tests covering these points:
— part of the operand of a sizeof operator whose result is an integer constant;
— part of the controlling expression of a generic selection;
— or, part of the operand of any typeof operator whose result is not a variably modified type.

I did not see how to add tests for the following since they require just a type which I thought to be possible to achieve
with an undefined internal since for a type the first declaration is the definition, e.g. with typedef.
— part of the operand of an alignof operator whose result is an integer constant;
— part of the expression in a generic association that is not the result expression of its generic
selection; (*this one is also a type since the association map contains only pairs of type names and result expressions*)

Is there a way to test undefined internals for these cases?

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