[PM] [cleanup] Update doxygen comments to use the new style, add some
doxygen comments, make existing comments doxygen comments etc.

Also, switch commented-out debug helpers to #if-0-ed out debug helpers.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195783 91177308-0d34-0410-b5e6-96231b3b80d8