c++: Diagnose references to void in structured bindings [PR99650]

c++: Diagnose references to void in structured bindings [PR99650]

We ICE on the following testcase, because std::tuple_element<...,...>::type
is void and for structured bindings we therefore need to create
void & or void && which is invalid.  We created such REFERENCE_TYPE and
later ICEd in the middle-end.
The following patch fixes it by diagnosing that.

2021-03-23  Jakub Jelinek  <jakub@redhat.com>

	PR c++/99650
	* decl.c (cp_finish_decomp): Diagnose void initializers when
	using tuple_element and get.

	* g++.dg/cpp1z/decomp55.C: New test.