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.

gcc/cp/parser.cc

@@ -16189,6 +16189,8 @@ cp_parser_linkage_specification (cp_parser* parser, tree prefix_attr)
     linkage = get_identifier (TREE_STRING_POINTER (linkage));
 
   /* We're now using the new linkage.  */
+  unsigned saved_module = module_kind;
+  module_kind &= ~MK_ATTACH;
   push_lang_context (linkage);
 
   /* Preserve the location of the innermost linkage specification,
@@ -16235,6 +16237,7 @@ cp_parser_linkage_specification (cp_parser* parser, tree prefix_attr)
 
   /* We're done with the linkage-specification.  */
   pop_lang_context ();
+  module_kind = saved_module;
 
   /* Restore location of parent linkage specification, if any.  */
   parser->innermost_linkage_specification_location = saved_location;

gcc/testsuite/g++.dg/modules/lang-3_a.C

@@ -0,0 +1,17 @@
+// { dg-additional-options "-fmodules-ts -Wno-pedantic" }
+module;
+# 4 __FILE__ 1
+void Quux ();
+# 6 "" 2
+export module bob;
+// { dg-module-cmi bob }
+
+extern "C++" 
+{
+export void Bar () {}
+export void Quux ();
+void Baz () {}
+}
+
+// { dg-final { scan-assembler {_Z3Barv:} } }
+// { dg-final { scan-assembler {_Z3Bazv:} } }

gcc/testsuite/g++.dg/modules/lang-3_b.C

@@ -0,0 +1,18 @@
+// { dg-additional-options -fmodules-ts }
+import bob;
+
+void Foo () 
+{
+  Bar ();
+  Baz (); // { dg-error "was not declared" }
+  Quux ();
+}
+
+void Bar ();
+void Baz ();
+
+void Quux ()
+{
+  Bar ();
+  Baz ();
+}

gcc/testsuite/g++.dg/modules/lang-3_c.C

@@ -0,0 +1,12 @@
+// { dg-additional-options -fmodules-ts }
+module bob;
+
+void Foo () 
+{
+  Bar ();
+  Baz ();
+}
+
+extern "C++" void Bar ();
+extern "C++" void Baz ();
+