IR: Add a configuration point for MDNodeInfo::isEqual, NFC
IR: Add a configuration point for MDNodeInfo::isEqual, NFC

This commit has no functionality change, but it adds a configuration
point for MDNodeInfo::isEqual to allow custom uniquing of subclasses of
MDNode, minimizing the diff of a follow-up.

git-svn-id: 0ff597fd157e6f4fc38580e8d64ab130330d2411@266542 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit 44387cfe5f8cf7b0ac399eb27a271d307d3b033a)

apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm,apple/swift-llvm