[PM] [cleanup] Replace a reserved identifier "_Self" with the injected
class name. I think we're no longer using any compilers with
sufficiently broken ICN for this use case, but I'll watch the bots and
introduce a typedef without a reserved name if any yell at me.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195793 91177308-0d34-0410-b5e6-96231b3b80d8