c++: Fix ICE with structured binding initialized to incomplete array [PR97878]

c++: Fix ICE with structured binding initialized to incomplete array [PR97878]

We ICE on the following testcase, for incomplete array a on auto [b] { a }; without
giving any kind of diagnostics, with auto [c] = a; during error-recovery.
The problem is that we get too far through check_initializer and e.g.
store_init_value -> constexpr stuff can't deal with incomplete array types.

As the type of the structured binding artificial variable is always deduced,
I think it is easiest to diagnose this early, even if they have array types
we'll need their deduced type to be complete rather than just its element
type.

2021-02-05  Jakub Jelinek  <jakub@redhat.com>

	PR c++/97878
	* decl.c (check_array_initializer): For structured bindings, require
	the array type to be complete.

	* g++.dg/cpp1z/decomp54.C: New test.

(cherry picked from commit b229baa75ce4627d1bd38f2d3dcd91af1a7071db)