[String] [Foundation] perf: UTF8 String -> Data

[ianpartridge]

Currently, str.data(using:allowLossyConversion:) always bridges via
NSString.

As String is now natively UTF-8 we can fastpath this conversion in the
case where the user requests UTF-8 encoding.

A benchmark for this was previously added in #22648.

[ianpartridge]

@swift-ci please benchmark

[swift-ci]

Performance: -O

TEST	OLD	NEW	DELTA	RATIO
Improvement
String.data.Small	341	31	-90.9%	11.00x
String.data.LargeUnicode	471	87	-81.5%	5.41x
String.data.Medium	418	84	-79.9%	4.98x
String.data.Empty	83	29	-65.1%	2.86x

Performance: -Osize

TEST	OLD	NEW	DELTA	RATIO
Improvement
String.data.Small	359	32	-91.1%	11.22x
String.data.LargeUnicode	496	91	-81.7%	5.45x
String.data.Medium	434	87	-80.0%	4.99x
String.data.Empty	84	30	-64.3%	2.80x
DataAppendDataLargeToLarge	50400	37600	-25.4%	1.34x (?)

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
String.data.Small	363	36	-90.1%	10.08x
String.data.LargeUnicode	504	99	-80.4%	5.09x
String.data.Medium	426	90	-78.9%	4.73x
String.data.Empty	91	33	-63.7%	2.76x

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

[ianpartridge]

10x for small strings, niiiiiiiiiiice 😎

@itaiferber @milseman please review.

[milseman]

Sweet!

CC @Catfish-Man

[milseman]

Also, let's cherry-pick this onto the 5.1 branch

[ianpartridge]

@swift-ci please smoke test

[ianpartridge]

Please merge when you're ready - thanks.