Fix how cc1 command line options are mapped into FP options.

Canonicalize on storing FP options in LangOptions instead of
redundantly in CodeGenOptions.  Incorporate -ffast-math directly
into the values of those LangOptions rather than considering it
separately when building FPOptions.  Build IR attributes from
those options rather than a mix of sources.

We should really simplify the driver/cc1 interaction here and have
the driver pass down options that cc1 directly honors.  That can
happen in a follow-up, though.

Patch by Michele Scandale!
https://reviews.llvm.org/D80315