swift234

Mark Array.withUnsafeMutableBuffer as not escaping the array storage.

This is safe because the closure is not allowed to capture the array according
to the documentation of 'withUnsafeMutableBuffer' and the current implementation
makes sure that any such capture would observe an empty array by swapping self
with an empty array.

Users will get "almost guaranteed" stack promotion for small arrays by writing
something like:

  func testStackAllocation(p: Proto) {
    var a = [p, p, p]
    a.withUnsafeMutableBufferPointer {
      let array = $0
      work(array)
    }
  }

It is "almost guaranteed" because we need to statically be able to tell the size
required for the array (no unspecialized generics) and the total buffer size
must not exceed 1K.