[PM] Switch new pass manager from polymorphic_ptr to unique_ptr now that
it is available. Also make the move semantics sufficiently correct to
tolerate move-only passes, as the PassManagers *are* move-only passes.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@203391 91177308-0d34-0410-b5e6-96231b3b80d8