[ABI] Associated conformance defaults

[DougGregor]

Implement support for default associated conformance witnesses, as part of the default witness table, which includes:

• Moving associated conformance witnesses into the resilient witness table
• Identifying default associated conformance witnesses in the type checker. We warn when building a protocol resiliently where a the default for an associated type does not satisfy one of the conformance requirements placed on that associated type
• Emitting default associated conformance witnesses into the default witness table
• Adding a fun Evolution test trying to simulate the moral equivalent of adding Collection.SubSequence resiliently.

[DougGregor]

@swift-ci please test

[DougGregor]

@swift-ci please benchmark

[DougGregor]

@milseman this is the pull request that introduces a new warning into the build of the standard library:

swift/stdlib/public/core/StringProtocol.swift:17:17: warning: default type 'Slice<Self>' for associated type 'SubSequence' does not satisfy constraint 'Self.SubSequence': 'StringProtocol'
public protocol StringProtocol
                ^
swift/stdlib/public/core/Collection.swift:393:18: note: associated type 'SubSequence' has default type 'Slice<Self>' written here
  associatedtype SubSequence = Slice<Self> where SubSequence.Index == Index
                 ^             ~~~~~~~~~~~

We can add the conditional conformance of Slice to StringProtocol, if that's the right thing to do. We don't currently have a different way to suppress the warning (which only triggers when building code resiliently).

[swift-ci]

Build comment file:

Code size: -O

TEST	OLD	NEW	DELTA	RATIO
Regression
NibbleSort.o	14711	14999	+2.0%	0.98x

Code size: -Osize

TEST	OLD	NEW	DELTA	RATIO
Regression
NibbleSort.o	19831	20263	+2.2%	0.98x

Code size: Swift libraries

TEST	OLD	NEW	DELTA	RATIO
Regression
libswiftObjectiveC.dylib	16384	20480	+25.0%	0.80x
libswiftCoreGraphics.dylib	61440	65536	+6.7%	0.94x
libswiftCloudKit.dylib	102400	106496	+4.0%	0.96x

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

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