c++/modules: Allow imported references in constant expressions

Currently the streaming code uses TREE_CONSTANT to determine whether an
entity will have a definition that is interesting to stream out.  This
is not sufficient, however; we also need to write the definition of
references, since although not TREE_CONSTANT they can still be usable in
constant expressions.

As such this patch uses the existing decl_maybe_constant_var function
which correctly handles this case.

gcc/cp/ChangeLog:

	* module.cc (has_definition): Use decl_maybe_constant_var
	instead of TREE_CONSTANT.

gcc/testsuite/ChangeLog:

	* g++.dg/modules/cexpr-5_a.C: New test.
	* g++.dg/modules/cexpr-5_b.C: New test.

Signed-off-by: Nathaniel Shead <nathanieloshead@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>

gcc/cp/module.cc

@@ -11829,7 +11829,7 @@ has_definition (tree decl)
 	       since there's no TU to emit them in otherwise.  */
 	    return true;
 
-	  if (!TREE_CONSTANT (decl))
+	  if (!decl_maybe_constant_var_p (decl))
 	    return false;
 
 	  return true;

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

@@ -0,0 +1,13 @@
+// { dg-additional-options "-fmodules-ts" }
+// { dg-module-cmi M }
+
+export module M;
+
+int x = 123;
+void f() {}
+
+int& xr = x;
+auto& fr = f;
+
+constexpr int& cxr = xr;
+constexpr auto& cfr = fr;

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

@@ -0,0 +1,9 @@
+// { dg-additional-options "-fmodules-ts" }
+
+module M;
+
+constexpr auto& use_xr = xr;
+constexpr auto& use_fr = fr;
+
+static_assert(&cxr == &use_xr);
+static_assert(&cfr == &use_fr);