[stdlib] add two-operand version of max and use max2 in stdlib when appropriate.

This helps array append's performance by ~ 2x. The generic max with a variadic
argument creates a temporary array then iterates over the array.

rdar://17140639 rdar://17073827


Swift SVN r18764

stdlib/core/StringBuffer.swift

@@ -90,7 +90,7 @@ struct _StringBuffer {
 
     // Allocate storage
     self = _StringBuffer(
-      capacity: max(utf16Count, minimumCapacity),
+      capacity: max2(utf16Count, minimumCapacity),
       initialSize: utf16Count,
       elementWidth: isAscii ? 1 : 2)