Macro history (de-)serialization. Deserialization currently reads only the...

Macro history (de-)serialization. Deserialization currently reads only the latest macro definition. Needs more work.

Summary: Passes all tests (+ the new one with code completion), but needs a thorough review in part related to modules.

Reviewers: doug.gregor

Reviewed By: alexfh

CC: cfe-commits, rsmith

Differential Revision: http://llvm-reviews.chandlerc.com/D41

llvm-svn: 164610