Remove redundant option -menable-unsafe-fp-math.

There are currently two options that are used to tell the compiler to perform
unsafe floating-point optimizations:
'-ffast-math' and '-funsafe-math-optimizations'.

'-ffast-math' is enabled by default. It automatically enables the driver option
'-menable-unsafe-fp-math'.
Below is a table illustrating the special operations enabled automatically by
'-ffast-math', '-funsafe-math-optimizations' and '-menable-unsafe-fp-math'
respectively.

Special Operations -ffast-math	-funsafe-math-optimizations -menable-unsafe-fp-math
MathErrno	       0	         1	                    1
FiniteMathOnly         1 	         0                          0
AllowFPReassoc	       1         	 1                          1
NoSignedZero	       1                 1                          1
AllowRecip             1                 1                          1
ApproxFunc             1                 1                          1
RoundingMath	       0                 0                          0
UnsafeFPMath	       1                 0                          1
FPContract	       fast	         on	                    on

'-ffast-math' enables '-fno-math-errno', '-ffinite-math-only',
'-funsafe-math-optimzations' and sets 'FpContract' to 'fast'. The driver option
'-menable-unsafe-fp-math' enables the same special options than
'-funsafe-math-optimizations'. This is redundant.
We propose to remove the driver option '-menable-unsafe-fp-math' and use
instead, the setting of the special operations to set the function attribute
'unsafe-fp-math'. This attribute will be enabled only if those special
operations are enabled and if 'FPContract' is either 'fast' or set to the
default value.

Differential Revision: https://reviews.llvm.org/D135097