Fix isInSystemMacro in presence of macro and pasted token

When a warning is raised from the expansion of a system macro that
involves pasted token, there was still situations were they were not
skipped, as showcased by this issue:
https://bugzilla.redhat.com/show_bug.cgi?id=1472437

Differential Revision: https://reviews.llvm.org/D59413

llvm-svn: 360885