Make language option `GNUAsm` discoverable with `__has_extension` macro.

This can be used for better support of `-fno-gnu-inline-asm` builds.

rdar://problem/49540880

Reviewers: aaron.ballman, rsmith

Reviewed By: aaron.ballman

Subscribers: eraman, jkorous, dexonsmith, craig.topper, cfe-commits

Differential Revision: https://reviews.llvm.org/D61619

llvm-svn: 360625