c++: Fix ICE with ill-formed array list-initialization [PR93712]

c++: Fix ICE with ill-formed array list-initialization [PR93712]

My P0388R4 patch changed build_array_conv to create an identity
conversion at the start of the conversion chain and now we crash
in convert_like_real:

 7457     case ck_identity:
 7458       if (BRACE_ENCLOSED_INITIALIZER_P (expr))
 7459         {
 7460           int nelts = CONSTRUCTOR_NELTS (expr);
 7461           if (nelts == 0)
 7462             expr = build_value_init (totype, complain);
 7463           else if (nelts == 1)
 7464             expr = CONSTRUCTOR_ELT (expr, 0)->value;
 7465           else
 7466             gcc_unreachable ();  // HERE
 7467         }

in a test like this

  int f (int const (&)[2])
  { return f({1, "M"}); }

Instead of creating a ck_identity at the start of the conversion chain,
so that conv_get_original_expr can be used with a ck_aggr, let's set
u.expr for a ck_aggr, and adjust next_conversion not to try to see
what's next in the chain if it gets a ck_aggr.

2020-02-24  Marek Polacek  <polacek@redhat.com>

	PR c++/93712 - ICE with ill-formed array list-initialization.
	* call.c (next_conversion): Return NULL for ck_aggr.
	(build_aggr_conv): Set u.expr instead of u.next.
	(build_array_conv): Likewise.
	(build_complex_conv): Likewise.
	(conv_get_original_expr): Handle ck_aggr.

	* g++.dg/cpp0x/initlist-array11.C: New test.