[libc++] Skip logic for detecting C11 features when using_if_exists is supported

In the future, we'll want to rely exclusively on using_if_exists for this
job, but for now, only rely on it when the compiler supports that attribute.
That removes the possibility for getting the logic wrong.

Differential Revision: https://reviews.llvm.org/D108297

(cherry picked from commit ebc01bbc)