c++: Implement DR2351 - void{} [PR102820]

Here is an implementation of DR2351 - void{} - where void{} after
pack expansion is considered valid and the same thing as void().
For templates, if CONSTRUCTOR_NELTS is 0, the CONSTRUCTOR is not dependent
and we can return void_node right away, if it is dependent and contains
only packs, then it is potentially zero element and so we need to build
CONSTRUCTOR_IS_DEPENDENT CONSTRUCTOR, while if it contains any non-pack
elts, we can diagnose it right away.

2021-10-29  Jakub Jelinek  <jakub@redhat.com>

	PR c++/102820
	* semantics.c (maybe_zero_constructor_nelts): New function.
	(finish_compound_literal): Implement DR2351 - void{}.
	If type is cv void and compound_literal has no elements, return
	void_node.  If type is cv void and compound_literal might have no
	elements after expansion, handle it like other dependent compound
	literals.

	* g++.dg/cpp0x/dr2351.C: New test.

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