libstdc++: Optimize std::decay

libstdc++: Optimize std::decay

Define partial specializations of std::decay and its __decay_selector
helper so that remove_reference, is_array and is_function are not
instantiated for every type, and remove_extent is not instantiated for
arrays.

libstdc++-v3/ChangeLog:

	* include/std/type_traits (__decay_selector): Add partial
	specializations for array types. Only check for function types
	when not dealing with an array.
	(decay): Add partial specializations for reference types.