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

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I've made some minor changes to clean up the LIT test checks for this PR. I started to clean up the entire test, but it quickly became obvious that I should put that into a separate PR.

I will submit a new PR to clean up the entire fast-math.c test. I have an idea that I think will work, hard-coding the order of the math-related flags and using a separate prefix for each one plus negative checks. Then the test can just use "--check-prefixes=..." to put together the combinations that are expected. I think that will be more maintainable and more readable than trying to group checks in the more-or-less arbitrary way the test currently does.

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