[PCH] Fixed bug with preamble invalidation when overridden files change

When remapped files were changed, they would not always cause the preamble's PCH to be invalidated, because the remapped path didn't necessarily match the include path (e.g. slash direction -- this happens a lot on Windows). I fixed this by moving to a llvm::sys::fs::UniqueID-based map instead of comparing paths stringwise.

Differential Revision: http://reviews.llvm.org/D20137

llvm-svn: 269769