[PM] Update Clang to reflect LLVM r199095 which moves the core DomTree
algorithms and datastructures into the fully generic support library,
separating them (almost) entirely from the LLVM IR. This makes the
reliance on domtrees here *much* cleaner.

It might be worthwhile for someone to use extern templates and other
tools to sink a lot more of this code into the .cpp files instead of the
.h files, but leaving that for someone other than me.