the front-end (as far as the preprocessor goes), follow the usual logic of
inserting the (original include path) name into the predefines buffer. This
pushes the responsibility for handling this to PCH instead of the front-end.  In
PCH this requires being a little more clever when we diff the predefines
buffers.

Neither of these solutions are particularly great, I think what we eventually
should do is something like gcc where we insert a special marker to indicate the
PCH file, but then run the preprocessor as usual. This would be clearer and
would allow us to drop the overly clever predefines handling.

llvm-svn: 86806