Fixed a problem with #pragma push_macro/pop_macro implementation.

Fixed a problem with #pragma push_macro/pop_macro implementation.

Summary:
The problem was with the following sequence:
  #pragma push_macro("long")
  #undef long
  #pragma pop_macro("long")
in case when "long" didn't represent a macro.
Fixed crash and removed code duplication for #undef/pop_macro case. Added regression tests.

Reviewers: doug.gregor, klimek

Reviewed By: doug.gregor

CC: cfe-commits, chapuni
Differential Revision: http://llvm-reviews.chandlerc.com/D31

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@162845 91177308-0d34-0410-b5e6-96231b3b80d8