[X86] Make `x86intrin.h`, `immintrin.h` includable with `-fno-gnu-inline-asm`.

Currently `immintrin.h` includes `pconfigintrin.h` and `sgxintrin.h`
which contain inline assembly. It causes failures when building with the
flag `-fno-gnu-inline-asm`.

Fix by excluding functions with inline assembly when this extension is
disabled. So far there was no need to support `_pconfig_u32`,
`_enclu_u32`, `_encls_u32`, `_enclv_u32` on platforms that require
`-fno-gnu-inline-asm`. But if developers start using these functions,
they'll have compile-time undeclared identifier errors which is
preferrable to runtime errors.

rdar://problem/49540880

Reviewers: craig.topper, GBuella, rnk, echristo

Reviewed By: rnk

Subscribers: jkorous, dexonsmith, cfe-commits

Differential Revision: https://reviews.llvm.org/D61621

llvm-svn: 360630