Ignore contents of #if 0 blocks.

Summary:
Added stack of preprocessor branching directives, and ignore all tokens
inside #if 0 except for preprocessor directives.

Reviewers: klimek, djasper

Reviewed By: klimek

CC: cfe-commits

Differential Revision: http://llvm-reviews.chandlerc.com/D855

llvm-svn: 182658