c++: Fix up mangling ICE with void{} [PR106863]

c++: Fix up mangling ICE with void{} [PR106863]

We ICE on the following testcase during mangling, finish_compound_literal
returns for void{} void_node and the mangler doesn't handle it.
Handling void_node in the mangler seems problematic to me, because
we don't know for which case it has been created.
The following patch arranges to mangle it as other compound literals
with no operands, so it demangles as void{}, by returning a void type
COMPOUND_LITERAL_P with no elements if processing_template_decl.
Otherwise it keeps returning void_node.

2022-10-21  Jakub Jelinek  <jakub@redhat.com>

	PR c++/106863
	* semantics.cc (finish_compound_literal): For void{}, if
	processing_template_decl return a COMPOUND_LITERAL_P
	CONSTRUCTOR rather than void_node.

	* g++.dg/cpp0x/dr2351-2.C: New test.