Tidy up PMStack. Add a bunch of consts, use std::vector instead of

Tidy up PMStack. Add a bunch of consts, use std::vector instead of
std::deque, since this is a stack and only supports push/pop on
one end, and remove an unimplemented declaration.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@110495 91177308-0d34-0410-b5e6-96231b3b80d8