Fix a variety of minor issues with ObjC method mangling:

- Fix a memory leak accidentally introduced yesterday by using CodeGen's
  existing mangling context instead of creating a new context afresh.

- Move GNU-runtime ObjC method mangling into the AST mangler; this will
  eventually be necessary to support direct methods there, but is also
  just the right architecture.

- Make the Apple-runtime method mangling work properly when given an
  interface declaration, fixing a bug (which had solidified into a test)
  where mangling a category method from the interface could cause it to
  be mangled as if the category name was a class name.  (Category names
  are namespaced within their class and have no global meaning.)

- Fix a code cross-reference in dsymutil.

Based on a patch by Ellis Hoag.