Clang importer: clean up importing of subscripts.

There was a lot of dodgy code in here that made subscript imports not
necessarily idempotent. Try to clean up the flow a bit, eliminating
the DeclContext parameter (which caused some "interesting" effects if
misused), be more methodical about handling the redeclaration case,
eliminate some unnecessary recursion due to performing name lookup in
the wrong place for override checking, etc.

Author: DougGregor