Support __seg_fs and __seg_gs on x86

Summary:
GCC supports named address spaces macros:
  https://gcc.gnu.org/onlinedocs/gcc/Named-Address-Spaces.html

clang does as well with address spaces:
  https://clang.llvm.org/docs/LanguageExtensions.html#memory-references-to-specified-segments

Add the __seg_fs and __seg_gs macros for compatibility with GCC.

<rdar://problem/52944935>

Subscribers: jkorous, dexonsmith, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D64676

llvm-svn: 366028