Streamline `Level::from_{attr,symbol}`

These methods return `Option<(Self, Option<LintExpectationId>)>`. But
all the call sites except one don't look at the
`Option<LintExpectationId>`.

This commit simplifies these methods to not return the
`Option<LintExpectationId>`. This means they no longer need to be passed
a closure to compute an `AttrId` (which is usually discarded anyway).
The commit also renames `from_attr` as `from_opt_symbol`, because it
takes an `Option<Symbol>`, not an `Attribute`.

These changes simplify all the call sites that don't need the
`Option<LintExpectationId>`, and also the one call site that does (in
`LintLevelsBuilder::add`): that call site no longer needs to do an
awkward destructuring, and can instead build the appropriate
`LintExpectationId` directly.