Revert "Reland D74436 "Change clang option -ffp-model=precise to select ffp-contract=on"""

Reverting because this patch is causing ~20 llvm-test-suite failures on
a number of different bots:
* http://lab.llvm.org:8011/builders/clang-cmake-armv8-lld/builds/3366
* http://lab.llvm.org:8011/builders/clang-cmake-aarch64-lld/builds/8222
* http://lab.llvm.org:8011/builders/clang-cmake-x86_64-avx2-linux/builds/13275
* http://lab.llvm.org:8011/builders/clang-s390x-linux-lnt/builds/17213

This reverts commit cd2c5af6.