c++: Fix decltype(auto) deduction with rvalue ref [PR78209]

Here's a small deficiency in decltype(auto).  [dcl.type.auto.deduct]/5:
If the placeholder-type-specifier is of the form decltype(auto), [...]
the type deduced for T is determined [...] as though E had been the operand
of the decltype.  So:

  int &&i = 0;
  decltype(auto) j = i; // should behave like int &&j = i; error

We deduce j's type in do_auto_deduction via finish_decltype_type which
takes an 'id' argument.  Currently we compute 'id' as false, because
stripped_init is *i (a REFERENCE_REF_P).  But it seems to me we should
rather set 'id' to true here, by looking through the REFERENCE_REF_P,
so that finish_decltype_type DTRT.

gcc/cp/ChangeLog:

	PR c++/78209
	* pt.c (do_auto_deduction): If init is REFERENCE_REF_P, use its
	first operand.

gcc/testsuite/ChangeLog:

	PR c++/78209
	* g++.dg/cpp1y/decltype-auto1.C: New test.

gcc/cp/pt.c

@@ -29278,6 +29278,8 @@ do_auto_deduction (tree type, tree init, tree auto_node,
   else if (AUTO_IS_DECLTYPE (auto_node))
     {
       tree stripped_init = tree_strip_any_location_wrapper (init);
+      if (REFERENCE_REF_P (stripped_init))
+	stripped_init = TREE_OPERAND (stripped_init, 0);
       bool id = (DECL_P (stripped_init)
 		 || ((TREE_CODE (init) == COMPONENT_REF
 		      || TREE_CODE (init) == SCOPE_REF)

gcc/testsuite/g++.dg/cpp1y/decltype-auto1.C

+// PR c++/78209
+// { dg-do compile { target c++14 } }
+
+int main()
+{
+  int &&i = 0;
+  decltype(auto) j = i; // { dg-error "cannot bind rvalue reference" }
+}