Change the signature for DispatchData.enumerateBytes() to match that …

[ktopley-apple]

…of Data.enumerateBytes() for Swift 4.2 and obsolete the old version.

rdar://problem/40775762

Adds overload DispatchData.enumerateBytes(_:) to match the signature used in Data and make it available as of Swift 4.2. Also obsoletes the existing version.

Resolves SR-7868.

[ktopley-apple]

@swift-ci please test

[ktopley-apple]

This is a cherry-pick of the change in #17982 on swift master back to swift-4.2-branch.

[ktopley-apple]

@swift-ci please nominate

Explanation: adds a new variant of DispatchData.enumerateBytes() for Swift 4.2 that matches the method in Data so that a single protocol can be used to manipulate objects of both types. The existing method is deprecated.
Scope: change to the API of DIspatchData
Radar/SR reference: rdar://problem/40775762, https://bugs.swift.org/browse/SR-7868.
Risk: Low. Code that uses the existing method will continue to work; on rebuild, Xcode should offer a fix-up for the obsolete method.
Testing: Tests have been added to verify that the correct version of the method is visible in Swift 4.2 and before, and to check that invocations that uses of the trailing block syntax are unaffected.