[modules] If both a module file and a module map for the same module are

explicitly provided, and the module map lists a header that does not exist,
unmark the module as 'unavailable' when loading its .pcm file. (Use of the
module might still fail if the relevant headers aren't embedded, but this
behavior is now consistent with how we behave if the module map is not
provided, and with the desired behavior for embedding headers in modules.)

llvm-svn: 264664