[PM] Rename the 'Mod' member to the more idiomatic 'M'. No functionality
changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195701 91177308-0d34-0410-b5e6-96231b3b80d8
[PM] Sink a trailing comment to be a doxygen comment.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@195702 91177308-0d34-0410-b5e6-96231b3b80d8