[clang][patch][fpenv] Add builtin __arithmetic_fence and option fprotect-parens

This patch adds a new clang builtin, __arithmetic_fence. The purpose of the
builtin is to provide the user fine control, at the expression level, over
floating point optimization when -ffast-math (-ffp-model=fast) is enabled.
The builtin prevents the optimizer from rearranging floating point expression
evaluation. The new option fprotect-parens has the same effect on
parenthesized expressions, forcing the optimizer to respect the parentheses.

Reviewed By: aaron.ballman, kpn

Differential Revision: https://reviews.llvm.org/D100118