c++: Allow exporting const-qualified namespace-scope variables [PR99232]

By [basic.link] p3.2.1, a non-template non-volatile const-qualified
variable is not necessarily internal linkage in a module declaration,
and rather may have module linkage (or external linkage if it is
exported, see p4.8).

	PR c++/99232

gcc/cp/ChangeLog:

	* decl.cc (grokvardecl): Don't mark variables attached to
	modules as internal.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/pr99232_a.C: New test.
	* g++.dg/modules/pr99232_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>

gcc/cp/decl.cc

@@ -11006,7 +11006,8 @@ grokvardecl (tree type,
 			    && (DECL_THIS_EXTERN (decl)
 				|| ! constp
 				|| volatilep
-				|| inlinep));
+				|| inlinep
+				|| module_attach_p ()));
       TREE_STATIC (decl) = ! DECL_EXTERNAL (decl);
     }
   /* Not at top level, only `static' makes a static definition.  */

gcc/testsuite/g++.dg/modules/pr99232_a.C

@@ -0,0 +1,12 @@
+// PR c++/99232
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi pr99232 }
+
+export module pr99232;
+
+export const double lambda{ 1.3 };
+export constexpr int a = 42;
+
+export const double* get_lambda_addr() {
+  return λ
+}

gcc/testsuite/g++.dg/modules/pr99232_b.C

@@ -0,0 +1,13 @@
+// PR c++/99232
+// { dg-module-do run }
+// { dg-additional-options "-fmodules-ts" }
+
+import pr99232;
+
+double foo() { return lambda * 2.0; }
+static_assert(a == 42);
+
+int main() {
+  if (&lambda != get_lambda_addr())
+    __builtin_abort();
+}