libstdc++: Remove __is_referenceable helper

We only use the __is_referenceable helper in three places now:
add_pointer, add_lvalue_reference, and add_rvalue_reference. But lots of
other traits depend on add_[lr]value_reference, and decay depends on
add_pointer, so removing the instantiation of __is_referenceable helps
compile all those other traits slightly faster.

We can just use void_t<T&> to check for a referenceable type in the
add_[lr]value_reference traits.

Then we can specialize add_pointer for reference types, so that we don't
need to use remove_reference, and then use void_t<T*> for all
non-reference types to detect when we can form a pointer to the type.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__is_referenceable): Remove.
	(__add_lvalue_reference_helper, __add_rvalue_reference_helper):
	Use __void_t instead of __is_referenceable.
	(__add_pointer_helper): Likewise.
	(add_pointer): Add partial specializations for reference types.

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