c++: follow DR 2386 and update implementation of get_tuple_size [PR110216]

DR 2386 updated the tuple_size requirements for structured binding and
it now requires tuple_size to be considered only if
std::tuple_size<TYPE> names a complete class type with member value. GCC
before this patch does not follow the updated requrements, and this
patch is intended to implement it.

(jason) Accepting pseudonym sign-off because a change this small is not
legally significant for copyright.

	DR 2386
	PR c++/110216

gcc/cp/ChangeLog:

	* decl.cc (get_tuple_size): Update implementation for DR 2386.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/decomp10.C: Update expected error for DR 2386.
	* g++.dg/cpp1z/pr110216.C: New test.

Signed-off-by: gnaggnoyil <gnaggnoyil@gmail.com>
Reviewed-by: Jason Merrill <jason@redhat.com>