Added backticks in a comment.
Added backticks in a comment.

`ObjectIdentifier` refers to a specific Swift structure. To make this clear, I have added backticks around it's occurrence in a comment.
apple/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,yoonapps/swift-corelibs-xctest,apple/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest,tiagomartinho/swift-corelibs-xctest