Fix isInSystemMacro to handle pasted macros

Token pasted by the preprocessor (through ##) have a Spelling pointing to scratch buffer.
As a result they are not recognized at system macro, even though the pasting happened in
a system macro. Fix that by looking into the parent macro if the original lookup finds a
scratch buffer.

Differential Revision: https://reviews.llvm.org/D55782

This effectively fixes https://bugs.llvm.org/show_bug.cgi?id=35268,

llvm-svn: 352838