c++: Add make_temp_override generator functions

c++: Add make_temp_override generator functions

A common pattern before C++17 is the generator function, used to avoid
having to specify the type of a container element by using a function call
to get type deduction; for example, std::make_pair.  C++17 added class type
argument deduction, making generator functions unnecessary for many uses,
but GCC won't be written in C++17 for years yet.

gcc/cp/ChangeLog:

	* cp-tree.h (struct type_identity): New.
	(make_temp_override): New.
	* decl.c (grokdeclarator): Use it.
	* except.c (maybe_noexcept_warning): Use it.
	* parser.c (cp_parser_enum_specifier): Use it.
	(cp_parser_parameter_declaration_clause): Use it.
	(cp_parser_gnu_attributes_opt): Use it.
	(cp_parser_std_attribute): Use it.