swift234

[stdlib] += no longer appends array elements

+= only extends arrays with another sequence of the same element type.

Fixes <rdar://problem/17151420> The use of the overloaded += operator in
Swift is inconsistent and confusing with Arrays.

Note that this commits generated 3 new radars against the type checker:

  <rdar://problem/17751308>
  <rdar://problem/17750582>
  <rdar://problem/17751359>

Swift SVN r20274