unique_ptr-ify ownership of ASTConsumers

(after fixing a bug in MultiplexConsumer I noticed the ownership of the
nested consumers was implemented with raw pointers - so this fixes
that... and follows the source back to its origin pushing unique_ptr
ownership up through there too)

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@213307 91177308-0d34-0410-b5e6-96231b3b80d8