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

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> 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.

Given: `_Generic(foo, bar : baz)`, `foo` is the controlling expression, `bar` is the generic association, and assuming `foo` is of type `bar`, then `baz` is the result expression of the generic selection.

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