[NFC] Extract method to SourceManager for traversing the macro "stack"

The code for going up the macro arg expansion is duplicated in many
places (and we need it for the analyzer as well, so I did not want to
duplicate it two more times).

This patch is an NFC, so the semantics should remain the same.

Differential Revision: https://reviews.llvm.org/D42458

llvm-svn: 324780