[driver] Improve the implementation of the -Ofast option.

Specifically, allow the flags that fall under this umbrella (i.e., -O3,
-ffast-math, and -fstrict-aliasing) to be overridden/disabled with the
individual -O[0|1|2|s|z]/-fno- flags.

This also fixes the handling of various floating point optimization
flags that are modified by -ffast-math (and thus -Ofast as well).
Part of rdar://13622687