Intrin.h: Clean up our atomic intrinsics

Three things:
- The atomic intrinsics mandate memory barriers, let's start emitting
  some.
- We don't need to manually create RMW operations, we can just do
  __atomic_fetch_foo instead of performing __atomic_foo_fetch and
  undoing foo.
- Don't use inline assembly, we don't need it for these intrinsics.

This fixes PR24101.

llvm-svn: 242009