[stdlib] Factor out Array growth calculation

This enables testing of other growth factors, per
<rdar://problem/17230502>.  See the radar for basic performance test
results confirming (to the degree practical) that 2x growth is better
than 1.5x in our current system.

Swift SVN r18990

stdlib/core/ContiguousArrayBuffer.swift

@@ -315,8 +315,10 @@ func += <
     (lhs.elementStorage + oldCount).initializeFrom(rhs)
   }
   else {
-    let newLHS = ContiguousArrayBuffer<T>(count: newCount, 
-                                      minimumCapacity: lhs.capacity * 2)
+    let newLHS = ContiguousArrayBuffer<T>(
+      count: newCount, 
+      minimumCapacity: _growArrayCapacity(lhs.capacity))
+    
     if lhs._base {
       newLHS.elementStorage.moveInitializeFrom(lhs.elementStorage, 
                                                count: oldCount)