swift234

stdlib: Implement Dictionary using a structs of arrays strategy

Instead of Optional entries use a third bitmap array to mark whether an entry is
valid. This representation saves space (because individual entries don't have a
alignment padding slack) and simplifies ARC's job leading to better performance.

Speedup measured (before/after) at -O:

DictionaryHashableClass`````````3.1
DollarFilter````````````````````1.3
DollarMap```````````````````````1.3
DollarReduce````````````````````1.4
JSONHelperDeserialize```````````1.4
NSDictionaryImplicitConversion``1.4
NSXMLParser`````````````````````1.3

Speedup measured (before/after) at -Onone:

DictOfArraysToArrayOfDicts``````1.3
Dictionary``````````````````````3.5
Dictionary2`````````````````````3.2
Dictionary3`````````````````````3.2
DictionaryHashableClass`````````1.9
DictionaryHashableStruct````````2.4
DictionaryRemove````````````````3.3
DictionarySwap``````````````````2.5
DollarFilter````````````````````1.2
DollarMap```````````````````````1.2
DollarReduce````````````````````1.3
Forest``````````````````````````1.8
Histogram```````````````````````1.6
JSONHelperDeserialize```````````1.6
NSDictionaryImplicitConversion``1.3
Prims```````````````````````````2.9
RGBHistogram````````````````````1.4

rdar://22173734

Swift SVN r32244