Teach the search for modules to consider modules described by a module

map, so long as they have an umbrella header. This makes it possible
to introduce a module map + umbrella header for a given set of
headers, to turn it into a module.

There are two major deficiencies here: first, we don't go hunting for
module map files when we just see a module import (so we won't know
about the modules described therein). Second, we don't yet have a way
to build modules that don't have umbrella headers, or have incomplete
umbrella headers.

llvm-svn: 144424