[PATCH] Support for modular module-map-files

[Daniel Jasper]

Hi doug.gregor, rsmith,

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").

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

Files:
  docs/Modules.rst
  lib/Lex/ModuleMap.cpp
  test/Modules/Inputs/modular_maps/a.h
  test/Modules/Inputs/modular_maps/b.h
  test/Modules/Inputs/modular_maps/module.map
  test/Modules/Inputs/modular_maps/moduleb.map
  test/Modules/modular_maps.cpp
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