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

[ktopley-apple]

@swift-ci please test

[ktopley-apple]

This is a cherry-pick of the change in #385 on master back to the 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 use trailing block syntax are unaffected.