I'm removing empty identifiers everywhere, because in practice they
always mean "no identifier" rather than "empty identifier". (An empty
identifier is impossible.) It's better to use `Option` to mean "no
identifier" because you then can't forget about the "no identifier"
possibility.
Some specifics:
- When testing an attribute for a single name, the commit uses the
`has_name` method.
- When testing an attribute for multiple names, the commit uses the new
`has_any_name` method.
- When using `match` on an attribute, the match arms now have `Some` on
them.
In the tests, we now avoid printing empty identifiers by not printing
the identifier in the `error:` line at all, instead letting the carets
point out the problem.
This shows places where the use of `name_or_empty` causes problems, i.e.
we print empty identifiers in error messages:
```
error: unrecognized field name ``
error: `` isn't a valid `#[macro_export]` argument
`#[no_sanitize()]` should be applied to a function
```
(The last one is about an attribute `#[no_sanitize("address")]`.)
The next commit will fix these.
