Preserve unknown STDC pragma through preprocessor
#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.
Reviewers: efriedma, rsmith, arphaman
Reviewed By: efriedma
Differential Revision: https://reviews.llvm.org/D41780