PTH:

- Added a side-table per each token-cached file with the preprocessor conditional stack.  This tracks what #if's are matched with what #endifs and where their respective tokens are in the PTH file.  This will allow for quick skipping of excluded conditional branches in the Preprocessor.
- Performance testing shows the addition of this information (without actually utilizing it) leads to no performance regressions.

llvm-svn: 60911