c++: decltype of capture proxy of ref [PR115504]

c++: decltype of capture proxy of ref [PR115504]

The finish_decltype_type capture proxy handling added in r14-5330 was
incorrectly stripping references in the type of the captured variable.

	PR c++/115504

gcc/cp/ChangeLog:

	* semantics.cc (finish_decltype_type): Don't strip the reference
	type (if any) of a capture proxy's captured variable.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1y/decltype-auto8.C: New test.

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