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.

---
This is a recommit of r299027 with an adjustment to the test
CodeGenCUDA/fp-contract.cu.  The test assumed that even
though -ffp-contract=on is passed FE-based folding of FMA won't happen.

This is obviously wrong since the user is asking for this explicitly with the
option.  CUDA is different that -ffp-contract=fast is on by default.

The test used to "work" because contract=fast and contract=on were maintained
separately and we didn't fold in the FE because contract=fast was on due to
the target-default.  This patch consolidates the contract=on/fast/off state
into a ternary state hence the change in behavior.
---

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

llvm-svn: 299033