c++: Modules Is Landing

This is the C++ modules implementation.  Dropping in the real
module.cc file into the module-shaped hole I carved out.  I include
some sanity tests for now.  I'll commit the remaining tests when the
initial round of failures seems abated.

	gcc/cp/
	* module.cc: Replace stubs with implementation.
	gcc/testsuite/
	* g++.dg/modules/mod-decl-0-2a.C: New.
	* g++.dg/modules/mod-decl-0.C: New.
	* g++.dg/modules/mod-decl-1.C: New.
	* g++.dg/modules/mod-decl-2_a.C: New.
	* g++.dg/modules/mod-decl-2_b.C: New.
	* g++.dg/modules/mod-decl-2_c.C: New.
	* g++.dg/modules/mod-decl-3.C: New.
	* g++.dg/modules/mod-decl-5_a.C: New.
	* g++.dg/modules/mod-decl-5_b.C: New.