c++: Support module language-decl semantics

c++: Support module language-decl semantics

In modules purview, one can attach a declaration to the global module
(i.e. not the named module in whence the declaration appears), using a
language declaration:

  export module Foo;
  extern "C++" void *operator new (std::size_t);

This implements those semantics.

	gcc/cp/
	* parser.cc (cp_parser_linkage_specification): Implement
	global module attachment semantics.
	gcc/testsuite/
	* g++.dg/modules/lang-3_a.C: New.
	* g++.dg/modules/lang-3_b.C: New.
	* g++.dg/modules/lang-3_c.C: New.