Fix ExistentialSpecializer: inherited conformance

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The ExistentialSpecializer incorrectly assumed that an existential's conformances match an opened archetype. They don't. Opened archetypes strip inherited conformances per the ABI for generic argument passing. Existential values retain those inherited conformances (for some inexplicable reason).

• Rename ASTContext::getExistentialSignature() to
  getOpenedArchetypeSiganture() because it was doing exactly the wrong
  thing for existentials.

• Fix ConcreteExistentialInfo to produce the correct SubstitutionMap.

• Fix ExistentialSpecializer to generate the correct conformances for
  init_existential by adding a collectExistentialConformances() helper.

Fixes rdar://problem/57025861 "Assertion failed: (conformances.size() == numConformanceRequirements)" in ExistentialSpecializer on inlined code

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@swift-ci test

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@swift-ci test source compatibility

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@swift-ci benchmark

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Performance: -O

Regression	OLD	NEW	DELTA	RATIO
ArrayAppendGenericStructs	630	1250	+98.4%	0.50x (?)
Improvement	OLD	NEW	DELTA	RATIO
DropLastArrayLazy	5	4	-20.0%	1.25x (?)
PolymorphicCalls	15	14	-6.7%	1.07x (?)

Code size: -O

Performance: -Osize

Improvement	OLD	NEW	DELTA	RATIO
PrefixWhileCountableRangeLazy	14	13	-7.1%	1.08x (?)

Code size: -Osize

Performance: -Onone

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 mini
  Model Identifier: Macmini8,1
  Processor Name: Intel Core i7
  Processor Speed: 3.2 GHz
  Number of Processors: 1
  Total Number of Cores: 6
  L2 Cache (per Core): 256 KB
  L3 Cache: 12 MB
  Memory: 64 GB

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Build failed
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Git Sha - 9a6a83bd8854d5888053a4c60993e0d665a4d70c

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Build failed
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Git Sha - 9a6a83bd8854d5888053a4c60993e0d665a4d70c

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@swift-ci test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 9a6a83bd8854d5888053a4c60993e0d665a4d70c

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 9a6a83bd8854d5888053a4c60993e0d665a4d70c

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@swift-ci smoke test

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@swift-ci smoke test