[clang] [clang] rename FPOptions.def's macro to FP_OPTION (PR #138374)

[Oliver Hunt via cfe-commits]

ojhunt wrote:

> LGTM, but I have only done a very isolated change on this code before.

Righto, I'll wait for @zahiraam and/or @AaronBallman as well

https://github.com/llvm/llvm-project/pull/138374