Recommit 213307: unique_ptr-ify ownership of ASTConsumers (reverted in r213325)

After post-commit review and community discussion, this seems like a
reasonable direction to continue, making ownership semantics explicit in
the source using the type system.
Recommit 213308: unique_ptr-ify ownership of ASTConsumers (reverted in r213324)

After post-commit review and community discussion, this seems like a
reasonable direction to continue, making ownership semantics explicit in
the source using the type system.