[stdlib] Make MutableCollection.partition(by:) implementation inlinable

[lorentey]

The default partition(by:) implementation cannot currently be specialized, which makes it ~64-100x slower than it could be for common cases. (E.g., Array<Int>.partition(by: { $0 >= foo}) is slower than .sort() by a factor of 2-32.)

Make the implementation inlinable.

[lorentey]

@swift-ci test

[lorentey]

@swift-ci benchmark

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 4f016c9

[lorentey]

@swift-ci test linux platform

[swift-ci]

Performance: -O

Regression	OLD	NEW	DELTA	RATIO
StringBuilderWithLongSubstring	1600	1770	+10.6%	0.90x (?)
Data.append.Sequence.809B.Count.RE.I	102	111	+8.8%	0.92x (?)
Improvement	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	6105	5448	-10.8%	1.12x (?)
Data.init.Sequence.511B.Count.I	97	89	-8.2%	1.09x (?)

Code size: -O

Regression	OLD	NEW	DELTA	RATIO
NibbleSort.o	14025	14441	+3.0%	0.97x

Performance: -Osize

Regression	OLD	NEW	DELTA	RATIO
Breadcrumbs.UTF16ToIdxRange.longASCII	30	34	+13.3%	0.88x
DictionaryBridgeToObjC_Access	814	901	+10.7%	0.90x (?)
Improvement	OLD	NEW	DELTA	RATIO
String.data.Empty	44	40	-9.1%	1.10x (?)
NSStringConversion.UTF8	1080	984	-8.9%	1.10x (?)

Code size: -Osize

Regression	OLD	NEW	DELTA	RATIO
NibbleSort.o	13827	14125	+2.2%	0.98x

Performance: -Onone

Regression	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeFromNSArrayAnyObjectForced	8180	9280	+13.4%	0.88x (?)
NSStringConversion.UTF8	3127	3425	+9.5%	0.91x (?)
Improvement	OLD	NEW	DELTA	RATIO
DictionaryBridgeToObjC_Access	1125	999	-11.2%	1.13x (?)
DataAppendArray	5700	5300	-7.0%	1.08x (?)

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

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 4f016c9

[natecook1000]

@swift-ci Please test Linux platform

[natecook1000]

LGTM!