RScanner iterates over all decls in our AST, but with modules we
have hidden decl from unimported submodules in our AST. As we
call Sema functions on these decls which use the normal clang
lookup that respects visibility, we suddenly get mysterious
lookup failures from inside Sema when running rootcling.

This patch restricts RScanner to looking at visible decls, which
restores the original behavior where RScanner onlys sees visible decls
from included headers.