Cleanup of OpaquePtr. No functionality changes.
- Some documenation were added.
- Usages of OpaquePtr<A>.getAsVal<A>() were replaced by OpaquePtr<A>.get().
- Methods getAs and getAsVal were renamed to getPtrTo and getPtrAs respectively.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@189346 91177308-0d34-0410-b5e6-96231b3b80d8