Module use declarations (II)

Review: http://llvm-reviews.chandlerc.com/D1546.

I have picked up this patch form Lawrence
(http://llvm-reviews.chandlerc.com/D1063) and did a few changes.

From the original change description (updated as appropriate):
This patch adds a check that ensures that modules only use modules they
have so declared. To this end, it adds a statement on intended module
use to the module.map grammar:

  use module-id

A module can then only use headers from other modules if it 'uses' them.
This enforcement is off by default, but may be turned on with the new
option -fmodules-decluse.

When enforcing the module semantics, we also need to consider a source
file part of a module. This is achieved with a compiler option

-fmodule-name=<module-id>.

The compiler at present only applies restrictions to the module directly
being built.