[clang] Clean up denormal handling with -ffp-model, -ffast-math, etc. (PR #89477)

[Andy Kaylor via cfe-commits]

andykaylor wrote:

I've updated this change to account for the changes made in https://github.com/llvm/llvm-project/pull/80475 and made corresponding updated to the PR title and description.

https://github.com/llvm/llvm-project/pull/89477