Use value semantics to manage DbgVariables rather than dynamic allocation/pointers.

Requires switching some vectors to lists to maintain pointer validity.
These could be changed to forward_lists (singly linked) with a bit more
work - I've left comments to that effect.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@206780 91177308-0d34-0410-b5e6-96231b3b80d8