Fix for expanding __pragmas in macro arguments

Summary:
Avoid parsing __pragma into an annotation token when macro arguments are pre-expanded.
This is what clang currently does when parsing _Pragmas.

Fixes https://bugs.llvm.org/show_bug.cgi?id=41128, where clang crashed
when trying to get the length of an annotation token.

Subscribers: cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D68114

llvm-svn: 373950