[stdlib] implement _copyContents for UnsafeRawBufferPointer

[glessard]

Implements _copyContents for Unsafe[Mutable]RawBufferPointer, which enables a fast path when filling up Array or UnsafeMutableBufferPointer<UInt8>.

Resolves SR-9604 (rdar://52529574).

[glessard]

Waiting on #38827 for benchmarking.

[glessard]

@swift-ci please benchmark

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
DictionaryOfAnyHashableStrings_insert	3192	5558	+74.1%	0.57x
Set.isDisjoint.Box25	355	505	+42.3%	0.70x (?)
Set.isDisjoint.Int50	266	335	+25.9%	0.79x (?)
StringFromLongWholeSubstring	4	5	+25.0%	0.80x
DictionaryKeysContainsNative	22	27	+22.7%	0.81x (?)
String.data.Medium	100	111	+11.0%	0.90x (?)
StringRemoveDupes	258	285	+10.5%	0.91x (?)
SetSubtractingInt50	111	120	+8.1%	0.93x (?)
DataToStringLargeUnicode	6200	6700	+8.1%	0.93x (?)
String.data.LargeUnicode	106	114	+7.5%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
RawBuffer.copyContents	914	48	-94.7%	19.04x
FlattenListFlatMap	6122	4353	-28.9%	1.41x (?)
DataAppendArray	4100	3800	-7.3%	1.08x (?)

Code size: -O

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
StringFromLongWholeSubstring	4	5	+25.0%	0.80x
DictionaryKeysContainsCocoa	26	28	+7.7%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
RawBuffer.copyContents	914	48	-94.7%	19.04x
NSStringConversion.Rebridge.UTF8	473	434	-8.2%	1.09x (?)

Code size: -Osize

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
DataToStringSmall	3900	4650	+19.2%	0.84x (?)
Improvement	OLD	NEW	DELTA	RATIO
RawBuffer.copyContents	914	48	-94.7%	19.04x

Code size: -swiftlibs

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

[glessard]

@swift-ci please smoke test

[atrick]

Wow, that's a pretty big missing performance feature.