Revert "unique_ptr-ify ownership of ASTConsumers"

This reverts commit r213308.

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