Allow resolving headers from a PCH even after headers+PCH were moved to another path.

Store in PCH the directory that the PCH was originally created in.
If a header file is not found at the path that we expect it to be and the PCH file
was moved from its original location, try to resolve the file by assuming that
header+PCH were moved together and the header is in the same place relative to the PCH.

llvm-svn: 125576