Add a 'RawPath' parameter to the PPCallbacks interface. This allows

clients to observe the exact path through which an #included file was
located. This is very useful when trying to record and replay inclusion
operations without it beind influenced by the aggressive caching done
inside the FileManager to avoid redundant system calls and filesystem
operations.

The work to compute and return this is only done in the presence of
callbacks, so it should have no effect on normal compilation.

Patch by Manuel Klimek.

llvm-svn: 127742