c++: fix ICE with is_really_empty_class [PR110106]

is_really_empty_class is liable to crash when it gets an incomplete
or dependent type.  Since r11-557, we pass the yet-uninstantiated
class type S<0> of the PARM_DECL s to is_really_empty_class -- because
of the potential_rvalue_constant_expression -> is_rvalue_constant_expression
change in cp_parser_constant_expression.  Here we're not parsing
a template so we did not check COMPLETE_TYPE_P as we should.

It should work to complete the type before checking COMPLETE_TYPE_P.

	PR c++/110106

gcc/cp/ChangeLog:

	* constexpr.cc (potential_constant_expression_1): Try to complete the
	type when !processing_template_decl.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/noexcept80.C: New test.

gcc/cp/constexpr.cc

@@ -9116,8 +9116,9 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       if (now && want_rval)
 	{
 	  tree type = TREE_TYPE (t);
-	  if ((processing_template_decl && !COMPLETE_TYPE_P (type))
-	      || dependent_type_p (type)
+	  if (dependent_type_p (type)
+	      || !COMPLETE_TYPE_P (processing_template_decl
+				   ? type : complete_type (type))
 	      || is_really_empty_class (type, /*ignore_vptr*/false))
 	    /* An empty class has no data to read.  */
 	    return true;

gcc/testsuite/g++.dg/cpp0x/noexcept80.C

@@ -0,0 +1,12 @@
+// PR c++/110106
+// { dg-do compile { target c++11 } }
+
+template<int> struct S
+{
+};
+
+struct G {
+  G(S<0>);
+};
+
+void y(S<0> s) noexcept(noexcept(G{s}));