[NFC] Refactor fast-math handling for clang driver (#81173)

This refactors the fast-math handling in the clang driver, moving the
settings into a lambda that is shared by the -ffp-model=fast and
-ffast-math code. Previously the -ffp-model=fast handler changed the
local option variable and fell through to the -ffast-math handler.

This refactoring is intended to prepare the way for decoupling the
-ffp-model=fast settings from the -ffast-math settings and possibly
introduce a less aggressive fp-model.