The handling of 'funsafe-math-optimizations' doesn't update the 'MathErrno'

flag. But the driver checks for 'fno-math-errno' before passing
'funsafe-math-optimizations' to the FE. In GCC, the option
'funsafe-math-optimizations' doesn't affect the 'fmath-errno' flag.
This patch aligns clang with GCC.

'-ffast-math' sets the FPContract to 'fast'. But 'funsafe-math-optimizations'
the driver doesn't consider the FPContract when handling the option.
Unfortunately there are places in the BE that interpret unsafe math
mode as allowing FMA. This patch makes -ffast-math' and
'funsafe-math-optimizations' behave similarly in regard to the setting of the
FPContract.

Differential Revision: https://reviews.llvm.org/D137578