Require @usableFromInline on associated type witnesses

[jrose-apple]

...when the protocol and the conforming type are not both public but are both public-or-usableFromInline. It's possible to write inlinable functions that depend on these types:

public protocol HasAssoc {
  associatedtype Assoc
}
public func getAssoc<T: HasAssoc>(_: T) -> T.Assoc

@usableFromInline struct Impl: HasAssoc {
  @usableFromInline typealias Assoc = Int
}

@inlinable func test() {
  let x: Int = getAssoc(Impl())
}

rdar://problem/43824052

[jrose-apple]

@swift-ci Please test

[jrose-apple]

@swift-ci Please test source compatibility

[jrose-apple]

@swift-ci Please benchmark

[lorentey]

Good catch!

[swift-ci]

Build comment file:

Performance: -Onone

TEST	OLD	NEW	DELTA	RATIO
Improvement
UTF8Decode_InitFromData_ascii	813	718	-11.7%	1.13x (?)

Code size: Swift libraries

TEST	OLD	NEW	DELTA	RATIO
Improvement
libswiftSwiftLang.dylib	81920	73728	-10.0%	1.11x

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

---

[jrose-apple]

@swift-ci Please test source compatibility