c++: consistently diagnose bare CTAD placeholder in fn return type

c++: consistently diagnose bare CTAD placeholder in fn return type

Relax slightly the existing check for diagnosing a bare CTAD placeholder
as the return type of a function declarator to also handle the abstract
declarator case.

gcc/cp/ChangeLog:

	* decl.cc (grokdeclarator): Diagnose a CTAD placeholder as
	function return type even when !funcdecl_p.

gcc/testsuite/ChangeLog:

	* g++.dg/other/pr88187.C: Adjust expected C++17 diagnostic.