Avoid CFAllocator lookup overhead in String bridging

[Catfish-Man]

Fixes rdar://problem/49925943

[Catfish-Man]

@swift-ci please smoke benchmark

[swift-ci]

Build failed before running benchmark.

[Catfish-Man]

@swift-ci please smoke benchmark

[swift-ci]

Build failed before running benchmark.

[Catfish-Man]

Yah. I should build it locally before running benchmarks apparently though :D

[Catfish-Man]

@swift-ci please smoke benchmark

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
FlattenListLoop	3979	4336	+9.0%	0.92x
RemoveWhereMoveInts	34	37	+8.8%	0.92x (?)
Array2D	6928	7520	+8.5%	0.92x (?)
RemoveWhereSwapInts	60	65	+8.3%	0.92x
MapReduce	368	397	+7.9%	0.93x (?)
MapReduceAnyCollection	369	398	+7.9%	0.93x
FlattenListFlatMap	6073	6550	+7.9%	0.93x (?)
Improvement
ObjectiveCBridgeToNSString	414	362	-12.6%	1.14x (?)
ObjectiveCBridgeFromNSString	1080	1005	-6.9%	1.07x (?)
ObjectiveCBridgeStubFromNSString	885	826	-6.7%	1.07x (?)

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
FlattenListLoop	4068	4431	+8.9%	0.92x (?)
RemoveWhereMoveInts	34	37	+8.8%	0.92x
Array2D	6912	7520	+8.8%	0.92x (?)
RemoveWhereSwapInts	62	67	+8.1%	0.93x
Improvement
ObjectiveCBridgeToNSString	417	365	-12.5%	1.14x (?)
EqualSubstringString	43	40	-7.0%	1.07x (?)
StringAdder	422	394	-6.6%	1.07x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
ObjectiveCBridgeToNSString	500	448	-10.4%	1.12x (?)
ArrayOfPOD	854	777	-9.0%	1.10x (?)
EqualStringSubstring	49	45	-8.2%	1.09x (?)
ObjectiveCBridgeStubFromNSString	923	859	-6.9%	1.07x (?)

How to read the data

The tables contain differences in performance which are larger than 8% and differences in code size which are larger than 1%.

If you see any unexpected regressions, you should consider fixing the
regressions before you merge the PR.

Noise: Sometimes the performance results (not code size!) contain false
alarms. Unexpected regressions which are marked with '(?)' are probably noise.
If you see regressions which you cannot explain you can try to run the
benchmarks again. If regressions still show up, please consult with the
performance team (@eeckstein).

Hardware Overview

  Model Name: Mac Pro
  Model Identifier: MacPro6,1
  Processor Name: 12-Core Intel Xeon E5
  Processor Speed: 2.7 GHz
  Number of Processors: 1
  Total Number of Cores: 12
  L2 Cache (per Core): 256 KB
  L3 Cache: 30 MB
  Memory: 64 GB

[Catfish-Man]

All the regressed tests have nothing to do with the code that changed, so I think this is a win

[Catfish-Man]

@swift-ci please smoke test

[Catfish-Man]

@swift-ci please smoke test

[milseman]

Please add an assertion in case someone tries to call it with a real allocator and intended it. Otherwise, LGTM

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge

[Catfish-Man]

@swift-ci please smoke test and merge