Revert "[clang][X86] Add __cpuidex function to cpuid.h"

This reverts commit 2df77ac2.

This has been causing some issues with some windows builds as
_MSC_EXTENSIONS isn't defined when only -fms-extensions is set, but the
builtin that conflicts with __cpuidex is. This was also causing problems
as it exposed some latent issues with how auxiliary triples are handled
in clang.

Differential Revision: https://reviews.llvm.org/D157115