[PM] [cleanup] Rearrange the public and private sections of this class
to be a bit more sensible. The public interface now is first followed by
the implementation details.

This also resolves a FIXME to make something private -- it was already
possible as the one special caller was already a friend.

No functionality changed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@196095 91177308-0d34-0410-b5e6-96231b3b80d8