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

[Joshua Cranmer via cfe-commits]

https://github.com/jcranmer-intel commented:

This may need some release notes adjustments as well; I already have a todo to revisit the release notes around release time to make sure we get the summary of the denormal handling flag changes right.

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