[clang] [llvm] [Analysis] Modernize CFGBlock GraphTraits; drop generic dominator tree special case (PR #207552)

[Alexis Engelke via llvm-commits]

https://github.com/aengelke approved this pull request.

LGTM. But please wait a bit in case folks who do more work on Clang have comments.

https://github.com/llvm/llvm-project/pull/207552