Preserve unknown STDC pragma through preprocessor

Summary:
#pragma STDC FP_CONTRACT handler is only registered in parser so we
should keep the unknown STDC pragma through preprocessor and we also
should not emit warning for unknown STDC pragma during preprocessor.

rdar://problem/35724351

Reviewers: efriedma, rsmith, arphaman

Reviewed By: efriedma

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41780

llvm-svn: 321909