Use FPContractModeKind universally

FPContractModeKind is the codegen option flag which is already ternary (off,
on, fast).  This makes it universally the type for the contractable info
across the front-end:

* In FPOptions (i.e. in the Sema + in the expression nodes).
* In LangOpts::DefaultFPContractMode which is the option that initializes
FPOptions in the Sema.

Another way to look at this change is that before fp-contractable on/off were
the only states handled to the front-end:
 * For "on", FMA folding was performed by  the front-end
 * For "fast", we simply forwarded the flag to TargetOptions to handle it in
 LLVM

Now off/on/fast are all exposed because for fast we will generate
fast-math-flags during CodeGen.

This is toward moving fp-contraction=fast from an LLVM TargetOption to a
FastMathFlag in order to fix PR25721.

Differential Revision: https://reviews.llvm.org/D31167

llvm-svn: 299027