[Driver] Clean up denormal handling with fast-math-related options (#89477)

This change cleans up the clang driver handling of umbrella options like
-ffast-math, -funsafe-math-optimizations, and -ffp-model, and aligns the
behavior of -ffp-model=fast with -ffast-math with regard to the linking
of crtfastmath.o.

We agreed in a previous review that the fast-math options should not
attempt to change the -fdenormal-fp-math option, which is inherently
target-specific.

The clang user's manual claims that -ffp-model=fast "behaves identically
to specifying both -ffast-math and -ffp-contract=fast." Since
-ffast-math causes crtfastmath.o to be linked if it is available, that should
also happen with -ffp-model=fast.

I am going to be proposing further changes to -ffp-model=fast,
decoupling it from -ffast-math and introducing a new
-ffp-model=aggressive that matches the current behavior, but I wanted
to solidfy the current behavior before I do that.