This applies the same changes from Clang CFE r349065 to the Swift
frontend to unify how filenames, cmpilation directories and absolute
paths in filenames and path remappings are handled.
This causes problems for cross-compilation -parse-stdlib tests that
emit debug info. At the moment we have zero of those, but we're
trying to add one.
Also, don't try to load new modules when recording imports. (This
isn't harmful, just inefficient.)
