SILGen: Emit literal closures at the abstraction level of their context.

[jckarter]

Literal closures are only ever directly referenced in the context of the expression they're written in,
so it's wasteful to emit them at their fully-substituted calling convention and then reabstract them if
they're passed directly to a generic function. Avoid this by saving the abstraction pattern of the context
before emitting the closure, and then lowering its main entry point's calling convention at that
level of abstraction. Generalize some of the prolog/epilog code to handle converting arguments and returns
to the correct representation for a different abstraction level.

[jckarter]

@swift-ci Please benchmark

[swift-ci]

Performance (x86_64): -O

Regression	OLD	NEW	DELTA	RATIO
DictionaryOfAnyHashableStrings_insert	3080	5600	+81.8%	0.55x
UnicodeStringFromCodable	508	727	+43.1%	0.70x (?)
Set.isDisjoint.Box25	355	508	+43.1%	0.70x (?)
LuhnAlgoEager	230	326	+41.7%	0.71x
LuhnAlgoLazy	229	321	+40.2%	0.71x (?)
Set.isDisjoint.Int25	268	343	+28.0%	0.78x (?)
DictionaryKeysContainsNative	22	25	+13.6%	0.88x (?)
DictionaryBridgeToObjC_Access	890	1008	+13.3%	0.88x (?)
StringRemoveDupes	262	293	+11.8%	0.89x (?)
SubstringFromLongString	10	11	+10.0%	0.91x (?)
ArrayInClass	1800	1970	+9.4%	0.91x (?)
FlattenListLoop	3889	4247	+9.2%	0.92x (?)
DistinctClassFieldAccesses	371	405	+9.2%	0.92x (?)
ObjectiveCBridgeToNSArray	12300	13400	+8.9%	0.92x (?)
XorLoop	1747	1890	+8.2%	0.92x (?)
ArrayPlusEqualFiveElementCollection	7807	8436	+8.1%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
ObjectiveCBridgeStubToNSDate2	600	560	-6.7%	1.07x (?)

Code size: -O

Regression	OLD	NEW	DELTA	RATIO
LuhnAlgoLazy.o	12162	13984	+15.0%	0.87x
LuhnAlgoEager.o	12162	13584	+11.7%	0.90x
Improvement	OLD	NEW	DELTA	RATIO
DictionaryRemove.o	13288	12912	-2.8%	1.03x
DictionarySwap.o	17113	16737	-2.2%	1.02x

Performance (x86_64): -Osize

Regression	OLD	NEW	DELTA	RATIO
FlattenListFlatMap	4306	6727	+56.2%	0.64x (?)
UnicodeStringFromCodable	508	714	+40.6%	0.71x (?)
FlattenListLoop	3976	5221	+31.3%	0.76x (?)
LuhnAlgoLazy	818	928	+13.4%	0.88x (?)
LuhnAlgoEager	817	925	+13.2%	0.88x (?)
ArrayInClass	1835	2010	+9.5%	0.91x (?)
Array2D	6928	7520	+8.5%	0.92x (?)
XorLoop	1747	1890	+8.2%	0.92x (?)
ArrayPlusEqualFiveElementCollection	7696	8325	+8.2%	0.92x (?)
ArrayAppendReserved	1370	1480	+8.0%	0.93x (?)
ArrayPlusEqualSingleElementCollection	1786	1927	+7.9%	0.93x (?)
RandomShuffleLCG2	416	448	+7.7%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
DropFirstAnySeqCRangeIterLazy	240	216	-10.0%	1.11x (?)
DropFirstAnySeqCntRangeLazy	240	216	-10.0%	1.11x (?)
PrefixAnyCollection	180	162	-10.0%	1.11x (?)
DropFirstAnyCollection	180	162	-10.0%	1.11x (?)
DropLastAnyCollection	63	57	-9.5%	1.11x (?)
DropWhileAnyCollection	198	180	-9.1%	1.10x (?)
DropFirstAnySeqCRangeIter	199	181	-9.0%	1.10x (?)
DropFirstAnySeqCntRange	199	181	-9.0%	1.10x (?)
PrefixAnySeqCRangeIterLazy	193	176	-8.8%	1.10x (?)
PrefixAnySeqCntRangeLazy	193	176	-8.8%	1.10x (?)
SuffixAnyCollection	69	63	-8.7%	1.10x (?)
DropWhileAnyCollectionLazy	275	252	-8.4%	1.09x (?)
ObjectiveCBridgeStubFromNSDate	6710	6160	-8.2%	1.09x (?)
DropWhileAnySeqCRangeIterLazy	305	281	-7.9%	1.09x (?)
DropWhileAnySeqCntRangeLazy	305	281	-7.9%	1.09x (?)
PrefixWhileAnyCollection	233	215	-7.7%	1.08x (?)
SuffixAnySequence	2569	2383	-7.2%	1.08x (?)
SuffixAnySequenceLazy	4896	4560	-6.9%	1.07x (?)

Code size: -Osize

Regression	OLD	NEW	DELTA	RATIO
LuhnAlgoLazy.o	13079	15999	+22.3%	0.82x
LuhnAlgoEager.o	13079	14779	+13.0%	0.88x
Improvement	OLD	NEW	DELTA	RATIO
DictionaryRemove.o	10579	10137	-4.2%	1.04x
DictionarySwap.o	15296	14854	-2.9%	1.03x
LazyFilter.o	7280	7177	-1.4%	1.01x
DictionaryCompactMapValues.o	12950	12819	-1.0%	1.01x

Performance (x86_64): -Onone

Regression	OLD	NEW	DELTA	RATIO
UnicodeStringFromCodable	534	747	+39.9%	0.71x (?)
DataAppendDataSmallToSmall	4520	5680	+25.7%	0.80x (?)
ArrayAppendOptionals	1360	1510	+11.0%	0.90x (?)
ObjectiveCBridgeStubToNSStringRef	263	289	+9.9%	0.91x (?)
String.replaceSubrange.String	25	27	+8.0%	0.93x (?)
Improvement	OLD	NEW	DELTA	RATIO
StringToDataSmall	3850	3200	-16.9%	1.20x (?)
StringToDataMedium	7900	6750	-14.6%	1.17x (?)
StringToDataEmpty	3550	3100	-12.7%	1.15x (?)
StringToDataLargeUnicode	8050	7400	-8.1%	1.09x (?)
MapReduceLazyCollectionShort	42191	39095	-7.3%	1.08x (?)
ObjectiveCBridgeFromNSDictionaryAnyObjectForced	12450	11550	-7.2%	1.08x (?)
String.data.Small	89	83	-6.7%	1.07x (?)

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

[jckarter]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 6542234f25b15dfb1ca6b20fc84132dd059f1761

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 6542234f25b15dfb1ca6b20fc84132dd059f1761

[jckarter]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - ae1193f4f985b14ba826fe1cd1bc09a09aa65ec2

[jckarter]

@swift-ci Please test

[jckarter]

@swift-ci Please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 309500d

[jckarter]

@swift-ci Please test macOS

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 309500d

[jckarter]

@swift-ci Please smoke test macOS