[clang] Fix -fno-unsafe-math-optimizations behavior (PR #89473)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I've made extensive changes to the fast-math.c test here: https://github.com/llvm/llvm-project/pull/89687

That update isn't intended to change any existing behavior. A few things worked differently than I expected, but I left the checks consistent with the current behavior and added comments where I thought the current behavior was incorrect.

https://github.com/llvm/llvm-project/pull/89473