Make NSObject.hashValue non-overridable

[lorentey]

This is cherry picked from @ikesyo's PR #1820 (commit 9ee6b02).

This PR fixes an NSObject API inconsistency between Swift 5's SDK overlays on Darwin and swift-corelibs-foundation. It also fixes several NSObject subclasses to customize hashing through the correct API, NSObject.hash.

NSObject.hashValue is currently declared open, which matches Darwin SDK overlays in Swift 4.2 and below. Unfortunately, hashValue was never supposed to be overridden -- NSObject subclasses are expected to customize hashing by overriding NSObject.hash, which is a distinct property. To help clear up the confusion resulting from this subtle gotcha, last year we decided to change NSObject.hashValue to be non-overridable.

• The Swift 4.2 compiler started emitting deprecation warnings for overrides of NSObject.hashValue ([Type checker] Warn about overrides of NSObject.hashValue. swift#18407).
• In Swift 5, the declaration of NSObject.hashValue was changed from @objc open to @nonobjc public. ([ObjectiveC] Make NSObject.hashValue non-overridable swift#20129)
• To help migrating existing (subtly broken) code, we plan to add a dedicated fix-it soon. ([Sema] Add dedicated fix-it for NSObject.hashValue overrides swift#22081)

[lorentey]

@swift-ci please test

[ikesyo]

Thanks for cherry-picking it.

[parkera]

Thanks @lorentey

[ianpartridge]

@swift-ci test and merge

[ianpartridge]

Thanks @lorentey - would you like to cherry-pick this to swift-5.0-branch?