DebugInfo: Write new DebugLoc API

Rewrite `DebugLoc` with a cleaner API that reflects its current status
as a wrapper around an `MDLocation` pointer.

  - Add accessors/constructors to/from `MDLocation`.
  - Simplify construction from `MDNode`.
  - Remove unnecessary `LLVMContext` from APIs.
  - Drop some API that isn't useful any more.
  - Rewrite documentation.

Actually, I've left the old API behind temporarily at the bottom of the
class so that I can update callers in separate commits.  I'll remove it
once the callers are updated.

llvm-svn: 233573