Support for modular module-map-files

This patch is the first step to make module-map-files modular (instead
of requiring a single "module.map"-file per include directory). This
step adds a new "extern module" declaration that enables
module-map-files to reference one another along with a very basic
implementation.

The next steps are:

* Combine this with the use-declaration (from
  http://llvm-reviews.chandlerc.com/D1546) in order to only load module
  map files required for a specific compilation.
* Add an additional flag to start with a specific module-map-file (instead
  of requiring there to be at least one "module.map").

Review: http://llvm-reviews.chandlerc.com/D1637