[5.0] Assorted String bridging improvements

[Catfish-Man]

Explanation:
• Convert _AbstractStringStorage to a protocol, and the free functions used to deduplicate implementations to extensions on that protocol.
• Move 'start' into the abstract type and use that to simplify some code
• Move the ASCII fast path for length into UTF16View.
• Add a weirder but faster way to check which (if any) of our NSString subclasses a given object is, and adopt it

Scope: This changes most operations on Swift Strings which are bridged into ObjC, but in relatively minor ways. This is a performance improvement, but is primarily important for Swift 5.0 because it will cause merge conflicts repeatedly if we don't have it.

Risk: Low-Medium. The main risk would be if someone is somehow relying on KVO subclassing of bridged Swift Strings, but since KVO doesn't actually work on NSStrings,

Testing: Passed regression tests, and the perf test suite exercises these operations over the bridge as well.

Reviewed by: @milseman

[milseman]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - dc6a85070c35aef89ef58d05136a602a42d794a3

[airspeedswift]

So #20900 supersedes this, right?

[milseman]

That cherry-picks this, which would otherwise conflict after either is merged. I don't care how you prefer to manage it, if you merge this than a rebase would drop it from the other PR.

The test failure looks legit:

/Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/stdlib/public/core/StringStorage.swift:191:25: error: property does not override any property from its superclass
09:06:32   override internal var count: Int {
09:06:32   ~~~~~~~~              ^
09:06:32 /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/stdlib/public/core/StringStorage.swift:83:16: warning: result of call to 'initialize(from:)' is unused
09:06:32         buffer.initialize(from: UnsafeBufferPointer(start: start, count: count))
09:06:32                ^         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
09:06:32 /Users/buildnode/jenkins/workspace/swift-PR-osx/branch-swift-5.0-branch/swift/stdlib/public/core/StringStorage.swift:135:15: error: pattern cannot match values of type '_KnownCocoaString'
09:06:32         case .tagged:
09:06:32              ~^~~~~~

@Catfish-Man, is there more to cherry-pick here? Can you diff the file vs master and make sure we get everything?

[Catfish-Man]

Dangit, I thought I included the fix for that. Will deal with that shortly.

[milseman]

@swift-ci please test

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - dc6a85070c35aef89ef58d05136a602a42d794a3

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - dc6a85070c35aef89ef58d05136a602a42d794a3

[Catfish-Man]

@swift-ci please test

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 98514e1a333ba8ffdddaf3b3cb432c61e7681ab6

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 98514e1a333ba8ffdddaf3b3cb432c61e7681ab6

[airspeedswift]

Closing as superseded by #20900