libstdc++: Optimize is_constructible traits

We can replace some class template helpers with alias templates, which
are cheaper to instantiate.

For example, replace the __is_copy_constructible_impl class template
with an alias template that uses just evaluates the __is_constructible
built-in, using add_lvalue_reference<const T> to get the argument type
in a way that works for non-referenceable types. For a given
specialization of is_copy_constructible this results in the same number
of class templates being instantiated (for the common case of non-void,
non-function types), but the add_lvalue_reference instantiations are not
specific to the is_copy_constructible specialization and so can be
reused by other traits. Previously __is_copy_constructible_impl was a
distinct class template and its specializations were never used for
anything except is_copy_constructible.

With the new definitions of these traits that don't depend on helper
classes, it becomes more practical to optimize the
is_xxx_constructible_v variable templates to avoid instantiations.
Previously doing so would have meant two entirely separate
implementation strategies for these traits.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__is_constructible_impl): Replace
	class template with alias template.
	(is_default_constructible, is_nothrow_constructible)
	(is_nothrow_constructible): Simplify base-specifier.
	(__is_copy_constructible_impl, __is_move_constructible_impl)
	(__is_nothrow_copy_constructible_impl)
	(__is_nothrow_move_constructible_impl): Remove class templates.
	(is_copy_constructible, is_move_constructible)
	(is_nothrow_constructible, is_nothrow_default_constructible)
	(is_nothrow_copy_constructible, is_nothrow_move_constructible):
	Adjust base-specifiers to use __is_constructible_impl.
	(__is_copy_assignable_impl, __is_move_assignable_impl)
	(__is_nt_copy_assignable_impl, __is_nt_move_assignable_impl):
	Remove class templates.
	(__is_assignable_impl): New alias template.
	(is_assignable, is_copy_assignable, is_move_assignable):
	Adjust base-specifiers to use new alias template.
	(is_nothrow_copy_assignable, is_nothrow_move_assignable):
	Adjust base-specifiers to use existing alias template.
	(__is_trivially_constructible_impl): New alias template.
	(is_trivially_constructible, is_trivially_default_constructible)
	(is_trivially_copy_constructible)
	(is_trivially_move_constructible): Adjust base-specifiers to use
	new alias template.
	(__is_trivially_assignable_impl): New alias template.
	(is_trivially_assignable, is_trivially_copy_assignable)
	(is_trivially_move_assignable): Adjust base-specifier to use
	new alias template.
	(__add_lval_ref_t, __add_rval_ref_t): New alias templates.
	(add_lvalue_reference, add_rvalue_reference): Use new alias
	templates.