c++: Fix excessive instantiation inside decltype [PR71879]

c++: Fix excessive instantiation inside decltype [PR71879]

Here after resolving the address of a template-id inside decltype, we
end up instantiating the chosen specialization (from the call to
mark_used in resolve_nondeduced_context), even though only its type is
needed.

This patch sets cp_unevaluated_operand throughout finish_decltype_type,
so that in particular it's set during the call to
resolve_nondeduced_context within.

gcc/cp/ChangeLog:

	PR c++/71879
	* semantics.c (finish_decltype_type): Set up a cp_unevaluated
	sentinel at the start of the function.  Remove a now-redundant
	manual adjustment of cp_unevaluated_operand.

gcc/testsuite/ChangeLog:

	PR c++/71879
	* g++.dg/cpp0x/decltype-71879.C: New test.