Summary: Support pragma FP_CONTRACT under strict FP.

Reviewers: craig.topper, andrew.w.kaylor, uweigand, RKSimon, LiuChen3

Subscribers: hiraditya, jdoerfert, cfe-commits, llvm-commits, LuoYuanke

Tags: #clang, #llvm

Differential Revision: https://reviews.llvm.org/D72820