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.

gcc/testsuite/g++.dg/modules/mod-decl-0-2a.C

@@ -0,0 +1,6 @@
+// { dg-options "-fno-modules -std=c++2a" }
+
+export // { dg-message "enabled with" }
+module nope; // { dg-error "not name a type" }
+// { dg-message "only available with .-fmodules." "" { target *-*-* } .-1 }
+// { dg-module-cmi "!nope" }

gcc/testsuite/g++.dg/modules/mod-decl-0.C

@@ -0,0 +1,6 @@
+// { dg-options "-fno-modules -std=c++17" }
+
+export // { dg-message "ignored" }
+module nope; // { dg-error "not name a type" }
+// { dg-message "only available with .-fmodules." "" { target *-*-* } .-1 }
+// { dg-module-cmi "!nope" }

gcc/testsuite/g++.dg/modules/mod-decl-1.C

@@ -0,0 +1,29 @@
+// { dg-additional-options "-fmodules-ts" }
+module;
+
+export module frist;
+// { dg-module-cmi "!frist" }
+
+import frist; // { dg-error {cannot import module.* in its own purview} }
+
+module foo.second; // { dg-error "not permitted here" }
+
+namespace Foo 
+{
+module third;  // { dg-error "not permitted here" }
+}
+
+struct Baz
+{
+  module forth; // { dg-error "expected" }
+};
+
+void Bink ()
+{
+  module fifth; // { dg-error "expected" }
+}
+
+module a.; // { dg-error "not permitted" }
+
+// { dg-prune-output "not writing module" }
+

gcc/testsuite/g++.dg/modules/mod-decl-2_a.C

@@ -0,0 +1,8 @@
+// { dg-additional-options "-fmodules-ts" }
+export module bob;
+// { dg-module-cmi "bob" }
+export void Foo ();
+export 
+{
+  void Bar ();
+}

gcc/testsuite/g++.dg/modules/mod-decl-2_b.C

@@ -0,0 +1,11 @@
+// { dg-additional-options "-fmodules-ts" }
+module bob;
+
+import bob; // { dg-error "cannot import module.* in its own purview" }
+
+// module linkage
+void Baz ()
+{
+  Foo ();
+  Bar ();
+}

gcc/testsuite/g++.dg/modules/mod-decl-2_c.C

@@ -0,0 +1,8 @@
+// { dg-additional-options "-fmodules-ts" }
+import bob;
+
+void Baz ()
+{
+  Foo ();
+  Bar ();
+}

gcc/testsuite/g++.dg/modules/mod-decl-3.C

@@ -0,0 +1,26 @@
+// { dg-additional-options "-fmodules-ts -Wno-pedantic" }
+module;
+# 4 "mod-decl-3.C" 1
+export void Foo (); // { dg-error "after a module interface" }
+
+# 7 "" 2
+export module bob;
+// { dg-module-cmi "!bob" }
+
+export
+export // { dg-error "occur once" }
+void Baz ();
+
+export 
+{
+  export // { dg-error "occur once" }
+    void Bar ();
+}
+
+namespace Bink
+{
+  import  // { dg-error "does not name" }
+  ben;
+}
+
+// { dg-prune-output "not writing module" }

gcc/testsuite/g++.dg/modules/mod-decl-5_a.C

@@ -0,0 +1,3 @@
+// { dg-additional-options "-fmodules-ts" }
+export module thing.baz;
+// { dg-module-cmi "thing.baz" }

gcc/testsuite/g++.dg/modules/mod-decl-5_b.C

@@ -0,0 +1,11 @@
+// { dg-additional-options "-fmodules-ts -Wno-pedantic" }
+module;
+# 4 "gmf" 1
+import thing.baz;
+
+export int foo (); // { dg-error "after a module interface" }
+# 8 "" 2
+export module thing.baz; // { dg-error "module already imported" }
+
+import thing.baz;
+