[clang] fast math test overhaul (PR #89687)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

> I think this works for the test. I'm slightly confused by the PR not-stacking with the test changes on top of the other PR

That's user error on my part. There are commits here that I didn't intend to be part of this PR. I'll clean it up before I submit it.

https://github.com/llvm/llvm-project/pull/89687