[dom] The formatting of the generic domtree has bitrotted over the years

[dom] The formatting of the generic domtree has bitrotted over the years
significantly. Clean it up with the help of clang-format.

I've touched this up by hand in a couple of places that weren't quite
right (IMO). I think most of these actually have bugs open about
already.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@225938 91177308-0d34-0410-b5e6-96231b3b80d8