[modules] If we import a module, and we've seen a module map that describes the
module, use the path from the module map file in preference to the path from
the .pcm file when resolving relative paths in the .pcm file. This allows
diagnostics (and .d output) to give relative paths if the module was found via
a relative path.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@223577 91177308-0d34-0410-b5e6-96231b3b80d8