Implement "-include-pth" in low-level driver. This allows a PTH file to be used

similar to a regular file passed to "-include". When -include-pth is used, the
low-level driver queries the PTH file for the name of the original source file
that generated the PTH file and implicitly adds a '#include' for that file in
the Predefines buffer.

llvm-svn: 67352