Revert "unique_ptr-ify ownership of ASTConsumers"

This reverts commit r213307.

Reverting to have some on-list discussion/confirmation about the ongoing
direction of smart pointer usage in the LLVM project.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213325 91177308-0d34-0410-b5e6-96231b3b80d8