Define __type_identity_t alias and use for string_view

Define __type_identity_t alias and use for string_view

This defines the equivalent of C++2a's std::type_identity_t alias but
for use in C++11 and later. This can be used to replace __detail::__idt
in the string_view headers, which previously used common_type_t because
the one argument specialization of common_type_t was simply the identity
transform (which is no longer true).

	* include/experimental/string_view (__detail::__idt): Remove.
	(operator==, operator!=, operator<, operator>, operator<=, operator>=):
	Use __type_identity_t instead of __detail::__idt;
	* include/std/string_view (__detail::__idt): Remove.
	(operator==, operator!=, operator<, operator>, operator<=, operator>=):
	Use __type_identity_t instead of __detail::__idt;
	* include/std/type_traits (__type_identity_t): New alias template.

From-SVN: r273442