[modules] When compiling a preprocessed module map, look for headers relative

to the original module map.

Also use the path and name of the original module map when emitting that
information into the .pcm file. The upshot of this is that the produced .pcm
file will track information for headers in their original locations (where the
module was preprocessed), not relative to whatever directory the preprocessed
module map was in when it was built.

llvm-svn: 304346