Let clang atomic builtins fetch add/sub support floating point types

Recently atomicrmw started to support fadd/fsub:

https://reviews.llvm.org/D53965

However clang atomic builtins fetch add/sub still does not support
emitting atomicrmw fadd/fsub.

This patch adds that.

Reviewed by: John McCall, Artem Belevich, Matt Arsenault, JF Bastien,
James Y Knight, Louis Dionne, Olivier Giroux

Differential Revision: https://reviews.llvm.org/D71726