swift234

[stdlib] Bring most of ArrayType into the stdlib

The ArrayType protocol is developed for NewArray is a good basis for
what we want, but the ability to create arrays without value semantics
is adding complexity but questionable value.  It isn't clear that we can
usefully expose a type called SharedArray<T>, even internally, if it
ceases to be truly shared when one copy grows, and therefore has its
buffer reallocated. Therefore, leave the NewArray test with a
ArrayType2, a refinement of ArrayType, that adds the ability to report
on its value semantics.

If we really want the optimizations that can be had in a large-scale
mutation by first unique'ing the array and making mutations in-place
thereafter, we can fall back to operating on NativeArrayBuffer<T>
directly.

Swift SVN r15673