[String] Naturalize Character

[milseman]

Characters should always be native, and never shared.

Resolves SR-9935.

[milseman]

@swift-ci please test

[milseman]

@swift-ci please benchmark

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
InsertCharacterEndIndexNonASCII	56	52	-7.1%	1.08x (?)

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
WordCount.o	44347	45195	+1.9%	0.98x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
NopDeinit	40000	47900	+19.7%	0.84x
ObjectiveCBridgeStubFromNSDateRef	3660	3970	+8.5%	0.92x (?)

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
WordCount.o	39580	40876	+3.3%	0.97x

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Regression
StringUTF16SubstringBuilder	17900	19470	+8.8%	0.92x (?)
Improvement
ArrayOfPOD	762	692	-9.2%	1.10x

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: 8-Core Intel Xeon E5
  Processor Speed: 3 GHz
  Number of Processors: 1
  Total Number of Cores: 8
  L2 Cache (per Core): 256 KB
  L3 Cache: 25 MB
  Memory: 16 GB

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 83e1137

[milseman]

@swift-ci please test linux platform

[milseman]

@swift-ci please test linux platform

[lorentey]

Looks good!