[clang-doc][NFC] Introduce OwnedPtr abstraction (#184869)
Eventually, we want clang-doc to support arena allocation, but the
widespread use of owning pointers in the data types prevents this.
Rather than have wide scale refactoring, we can introduce a type alias
that can be swapped out atomically to switch from smart pointers to raw
pointers. This is the first of several refactorings that are intended to
make the transition simpler.
