c++: decltype of capture proxy [PR79378, PR96917]

c++: decltype of capture proxy [PR79378, PR96917]

We typically don't see capture proxies in finish_decltype_type because
process_outer_var_ref is a no-op within an unevaluated context and so a
use of a captured variable within decltype resolves to the captured
variable, not the capture.  But we can see them during decltype(auto)
deduction and for decltype of an init-capture, which suggests we need to
handle capture proxies specially within finish_decltype_type after all.
This patch adds such handling.

	PR c++/79378
	PR c++/96917

gcc/cp/ChangeLog:

	* semantics.cc (finish_decltype_type): Handle an id-expression
	naming a capture proxy specially.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto7.C: New test.
	* g++.dg/cpp1y/lambda-init20.C: New test.

Reviewed-by: Jason Merrill <jason@redhat.com>