c++: check completeness after auto deduction [PR80351]

Normally we check for incomplete type in start_decl, but that obviously
doesn't work for auto variables. Thanks to Pokechu22 for the analysis and
testcases:

"When cp_finish_decl calls cp_apply_type_quals_to_decl on a const auto or
constexpr auto variable, the type might not be complete the first time
(this happened when auto deduces to an initializer_list).
cp_apply_type_quals_to_decl removes the const qualifier if the type is
not complete, which is appropriate for grokdeclarator, on the assumption
that the type will be complete when called by cp_finish_decl."

	PR c++/80351

gcc/cp/ChangeLog:

	* decl.cc (cp_finish_decl): Check completeness of deduced type.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp0x/constexpr-77482.C: Adjust message.
	* g++.dg/cpp1y/auto-fn27.C: Likewise.
	* g++.dg/cpp1y/lambda-generic-variadic22.C: Likewise.
	* g++.dg/cpp1z/decomp54.C: Likewise.
	* g++.dg/cpp0x/initlist-const1.C: New test.
	* g++.dg/warn/Wunused-var-37.C: New test.
	* g++.dg/warn/Wunused-var-38.C: New test.
	* g++.dg/warn/Wunused-var-39.C: New test.