swift234

[stdlib] Remove ArrayBuffer indirection

The case where Array stores class instances no longer requires an
intermediate indirect buffer object.

Also fixes <rdar://problem/17348939>

These are the speed changes < 0.95x and > 1.05x, as I measured them.
Although I don't have great confidence in these numbers, some are
consistent with Arnold's measurements, FWIW.

-O:
0.93 ArrayLiteral
1.56 Ary
1.39 Ary2
1.06 CaptureProp
1.93 ClassArrayGetter
1.08 DeltaBlue
1.08 DollarChain
1.13 InsertionSort
1.08 PrimeNum
1.11 RC4
0.93 Rectangles
1.08 SwiftStructuresBubbleSort

-Onone:
1.06 ArrayLiteral
1.10 ArraySubscript
1.12 Ary
1.12 Ary2
1.10 Ary3
1.20 ClassArrayGetter
1.19 DeltaBlue
1.08 DollarChain
1.09 Hash
1.07 Havlak
1.10 HeapSort
1.11 ImageProc
1.17 InsertionSort
1.18 Memset
1.11 NBody
1.07 PrimeNum
1.11 QuickSort
1.12 RC4
1.08 Rectangles
1.07 SelectionSort
1.06 StringBuilder
1.12 SwiftStructuresBubbleSort
1.10 Walsh
1.12 XorLoop

-Ounchecked:
0.91 ArrayLiteral
1.74 Ary
1.53 Ary2
2.08 ClassArrayGetter
1.19 DeltaBlue
1.06 DollarChain
1.07 Havlak
1.09 ImageProc
1.22 InsertionSort
0.89 PopFrontArrayGeneric
1.11 PrimeNum
0.85 QuickSort
1.10 RC4
1.12 Rectangles
1.06 StrToInt
1.11 SwiftStructuresBubbleSort

Swift SVN r23551