c++: fix ICE with designated initializer [PR110114]

c++: fix ICE with designated initializer [PR110114]

r13-1227 added an assert checking that the index in a CONSTRUCTOR
is a FIELD_DECL.  That's a reasonable assumption but in this case
we never called reshape_init due to the type being incomplete, and
so the index remained an identifier node: get_class_binding never
got around to looking up the FIELD_DECL.

We can avoid the crash by returning early in implicit_conversion_1; we'd
return NULL anyway due to:

  if (i < CONSTRUCTOR_NELTS (ctor))
    return NULL;

in build_aggr_conv.

	PR c++/110114

gcc/cp/ChangeLog:

	* call.cc (implicit_conversion_1): Return early if the type isn't
	complete.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/initlist100.C: Adjust expected diagnostic.
	* g++.dg/cpp2a/desig28.C: New test.
	* g++.dg/cpp2a/desig29.C: New test.