[Driver, CodeGen] add options to enable/disable an FP cast optimization

As discussed in the post-commit thread for:
rL330437 ( http://lists.llvm.org/pipermail/llvm-commits/Week-of-Mon-20180423/545906.html )

We need a way to opt-out of a float-to-int-to-float cast optimization because too much 
existing code relies on the platform-specific undefined result of those casts when the 
float-to-int overflows.

The LLVM changes associated with adding this function attribute are here:
rL330947
rL330950
rL330951

Also as suggested, I changed the LLVM doc to mention the specific sanitizer flag that 
catches this problem:
rL330958

Differential Revision: https://reviews.llvm.org/D46135

llvm-svn: 331041