c++: ICE initing lifetime-extended constexpr var [PR107079]

We ICE on the simple:

  struct X { const X* x = this; };
  constexpr const X& x = X{};

where store_init_value initializes 'x' with

  &TARGET_EXPR <D.2768, {.x=(const struct X *) &<PLACEHOLDER_EXPR struct X>}>

but we must lifetime-extend via extend_ref_init_temps and we get

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>;, (const struct X &) &_ZGR1x_;

Since 'x' was declared constexpr, we do cxx_constant_init and we hit
the preeval code added in r269003 while evaluating the INIT_EXPR:

  _ZGR1x_.x = (const struct X *) &<PLACEHOLDER_EXPR struct X> >>>

but we have no ctx.ctor or ctx.object here so lookup_placeholder won't
find anything that could replace X and we ICE on
 7861       /* A placeholder without a referent.  We can get here when
 7862          checking whether NSDMIs are noexcept, or in massage_init_elt;
 7863          just say it's non-constant for now.  */
 7864       gcc_assert (ctx->quiet);
because cxx_constant_init means !ctx->quiet.  It's not correct that
there isn't a referent.  I think the following patch is a pretty
straightforward fix: pass the _ZGR var down to maybe_constant_init so
that it can replace the PLACEHOLDER_EXPR with _ZGR1x_.

The commented assert in the test doesn't pass: we complain that _ZGR1x_
isn't a constexpr variable because we don't implement DR2126 (PR101588).

	PR c++/107079

gcc/cp/ChangeLog:

	* call.cc (set_up_extended_ref_temp): Pass var to maybe_constant_init.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-nsdmi2.C: New test.

gcc/cp/call.cc

@@ -13583,7 +13583,7 @@ set_up_extended_ref_temp (tree decl, tree expr, vec<tree, va_gc> **cleanups,
 
   /* If the initializer is constant, put it in DECL_INITIAL so we get
      static initialization and use in constant expressions.  */
-  init = maybe_constant_init (expr);
+  init = maybe_constant_init (expr, var, /*manifestly_const_eval=*/true);
   /* As in store_init_value.  */
   init = cp_fully_fold (init);
   if (TREE_CONSTANT (init))

gcc/testsuite/g++.dg/cpp0x/constexpr-nsdmi2.C

@@ -0,0 +1,9 @@
+// PR c++/107079
+// { dg-do compile { target c++11 } }
+
+struct X {
+  const X* x = this;
+};
+constexpr const X& x = X{};
+// TODO: The assert should pass once we implement DR2126 (c++/101588).
+//static_assert(x.x == &x);