Point at lint name instead of whole attr for gated lints

```
warning: unknown lint: `test_unstable_lint`
  --> $DIR/warn-unknown-unstable-lint-inline.rs:4:10
   |
LL | #![allow(test_unstable_lint, another_unstable_lint)]
   |          ^^^^^^^^^^^^^^^^^^
   |
   = note: the `test_unstable_lint` lint is unstable
   = help: add `#![feature(test_unstable_lint)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
note: the lint level is defined here
  --> $DIR/warn-unknown-unstable-lint-inline.rs:3:9
   |
LL | #![warn(unknown_lints)]
   |         ^^^^^^^^^^^^^

warning: unknown lint: `test_unstable_lint`
  --> $DIR/warn-unknown-unstable-lint-inline.rs:4:29
   |
LL | #![allow(test_unstable_lint, another_unstable_lint)]
   |                              ^^^^^^^^^^^^^^^^^^^^^
   |
   = note: the `test_unstable_lint` lint is unstable
   = help: add `#![feature(test_unstable_lint)]` to the crate attributes to enable
   = note: this compiler was built on YYYY-MM-DD; consider upgrading it if it is out of date
note: the lint level is defined here
  --> $DIR/warn-unknown-unstable-lint-inline.rs:3:9
   |
LL | #![warn(unknown_lints)]
   |         ^^^^^^^^^^^^^
```

This is particularly relevant when there are multiple lints in the same `warn` attribute. Pointing at the smaller span makes it clearer which one the warning is complaining about.

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