Forbid unnecessary visibility on view items

For `use` statements, this means disallowing qualifiers when in functions and
disallowing `priv` outside of functions.

For `extern mod` statements, this means disallowing everything everywhere. It
may have been envisioned for `pub extern mod foo` to be a thing, but it
currently doesn't do anything (resolve doesn't pick it up), so better to err on
the side of forwards-compatibility and forbid it entirely for now.

Closes #9957
auto merge of #11607 : alexcrichton/rust/issue-9957, r=cmr

For `use` statements, this means disallowing qualifiers when in functions and
disallowing `priv` outside of functions.

For `extern mod` statements, this means disallowing everything everywhere. It
may have been envisioned for `pub extern mod foo` to be a thing, but it
currently doesn't do anything (resolve doesn't pick it up), so better to err on
the side of forwards-compatibility and forbid it entirely for now.

Closes #9957