[PM][cleanup] Clean up comments and use modern doxygen in this file.
This is a precursor to breaking the pass that computes the DominatorTree
apart from the concrete DominatorTree.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@199103 91177308-0d34-0410-b5e6-96231b3b80d8