Add -ffp-contract = { fast | on | off } command line option support.

This flag sets the 'fp-contract' mode, which controls the formation of fused
floating point operations. Available modes are:

- Fast: Form fused operations anywhere. 
- On: Form fused operations where allowed by FP_CONTRACT. This is the default
      mode.
- Off: Don't form fused operations (in future this may be relaxed to forming
       fused operations where it can be proved that the result won't be
       affected).

Currently clang doesn't support the FP_CONTRACT pragma, so the 'On' and 'Off'
modes are equivalent.

llvm-svn: 159794