[clang][fpenv][patch] Change clang option -ffp-model=precise to select ffp-contract=on

Change the ffp-model=precise to enables -ffp-contract=on (previously
-ffp-model=precise enabled -ffp-contract=fast). This is a follow-up
to Andy Kaylor's comments in the llvm-dev discussion "Floating Point
semantic modes". From the same email thread, I put Andy's distillation
of floating point options and floating point modes into UsersManual.rst
Also fixes bugs.llvm.org/show_bug.cgi?id=50222

I had to revert this a few times because of failures on the x86-64
buildbot but I think we finally have that fixed by LNT/79f2b03c51.

Reviewed By: rjmccall, andrew.kaylor

Differential Revision: https://reviews.llvm.org/D74436