[AMDGPU] Expose llvm fence instruction as clang intrinsic

Expose llvm fence instruction as clang builtin for AMDGPU target

__builtin_amdgcn_fence(unsigned int memoryOrdering, const char *syncScope)

The first argument of this builtin is one of the memory-ordering specifiers
__ATOMIC_ACQUIRE, __ATOMIC_RELEASE, __ATOMIC_ACQ_REL, or __ATOMIC_SEQ_CST
following C++11 memory model semantics. This is mapped to corresponding
LLVM atomic memory ordering for the fence instruction using LLVM atomic C
ABI. The second argument is an AMDGPU-specific synchronization scope
defined as string.

Reviewed By: sameerds

Differential Revision: https://reviews.llvm.org/D75917