[Driver, CodeGen] pass through and apply -fassociative-math

There are 2 parts to getting the -fassociative-math command-line flag translated to LLVM FMF:

1. In the driver/frontend, we accept the flag and its 'no' inverse and deal with the 
   interactions with other flags like -ffast-math -fno-signed-zeros -fno-trapping-math. 
   This was mostly already done - we just need to translate the flag as a codegen option. 
   The test file is complicated because there are many potential combinations of flags here.
   Note that we are matching gcc's behavior that requires 'nsz' and no-trapping-math.

2. In codegen, we map the codegen option to FMF in the IR builder. This is simple code and 
   corresponding test.

For the motivating example from PR27372:

float foo(float a, float x) { return ((a + x) - x); }

$ ./clang -O2 27372.c -S -o - -ffast-math  -fno-associative-math -emit-llvm  | egrep 'fadd|fsub'
  %add = fadd nnan ninf nsz arcp contract float %0, %1
  %sub = fsub nnan ninf nsz arcp contract float %add, %2

So 'reassoc' is off as expected (and so is the new 'afn' but that's a different patch). 
This case now works as expected end-to-end although the underlying logic is still wrong:

$ ./clang  -O2 27372.c -S -o - -ffast-math  -fno-associative-math | grep xmm
	addss	%xmm1, %xmm0
	subss	%xmm1, %xmm0

We're not done because the case where 'reassoc' is set is ignored by optimizer passes. Example:

$ ./clang  -O2 27372.c -S -o - -fassociative-math -fno-signed-zeros -fno-trapping-math -emit-llvm  | grep fadd
  %add = fadd reassoc float %0, %1

$ ./clang -O2  27372.c -S -o - -fassociative-math -fno-signed-zeros -fno-trapping-math | grep xmm
	addss	%xmm1, %xmm0
	subss	%xmm1, %xmm0

Differential Revision: https://reviews.llvm.org/D39812

llvm-svn: 320920