Implement __cpuid and __cpuidex as Clang builtins

https://reviews.llvm.org/D23944 implemented the #pragma intrinsic from
MSVC. This causes the statement #pragma intrinsic(cpuid) to fail [0]
on Clang because cpuid is currently implemented in intrin.h instead
of a Clang builtin. Reimplementing cpuid (as well as it's releated
function, cpuidex) should resolve this.

[0]: https://crbug.com/1279344

Differential revision: https://reviews.llvm.org/D121653