Fix an logic error in the clang driver preventing crtfastmath.o from linking when -Ofast is used without -ffast-math

In gcc using -Ofast forces linking of crtfastmath.o.
In the current clang crtfastmath.o is only linked when -ffast-math/-funsafe-math-optimizations passed. It can lead to performance issues, when using only -Ofast without explicit -ffast-math (I faced with it).
My patch fixes inconsistency with gcc behaviour and also introduces few tests on it.

Patch by Zinovy Nis!

Differential Revision: http://llvm-reviews.chandlerc.com/D3114