Revert "Revert "Change clang option -ffp-model=precise to select ffp-contract=on""

This reverts commit 99c5bcbc.
Change clang option -ffp-model=precise to select ffp-contract=on
Including some small touch-ups to the original commit

Reviewers: rjmccall, Andy Kaylor

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