[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

stdlib/core/StringUTF8.swift

@@ -153,7 +153,7 @@ extension String {
     var result = ContiguousArray<UTF8.CodeUnit>()
     result.reserveCapacity(countElements(utf8) + 1)
     result += utf8
-    result += 0
+    result.append(0)
     return result
   }
 }