Mention invariant.group in ReleaseNotes
Mention invariant.group in ReleaseNotes

https://reviews.llvm.org/D28605



git-svn-id: 27e84c199cfb3049f109a3001c965f4b97b0bded@292009 91177308-0d34-0410-b5e6-96231b3b80d8

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