Fix a type (MAX_PATH instead of PATH_MAX) and remove an unused variable.
Now that we get types from modules, we occasionally
encounter clang::ExternalASTSources that are not instances
of ClangExternalASTSourceCommon.  We used to blithely
assume that all are, and so we could use static_cast<>.
That's no longer the case, so we have to have these AST
sources register themselves.