`#![no_start]` attribute emits false root module warning

[pnkfelix]

(Spawned off of issue #43106)

Compiling this code (in stable/beta/nightly) results in the following compiler warnings:

#![no_start]
fn main() { }

warnings:

warning: unused attribute
 --> <anon>:1:1
  |
1 | #![no_start]
  | ^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

warning: crate-level attribute should be in the root module
 --> <anon>:1:1
  |
1 | #![no_start]
  | ^^^^^^^^^^^^
  |
  = note: #[warn(unused_attributes)] on by default

Similarly to #43142, the warning that a crate-level attribute should be in the root module is false here: This attribute is in the root module.

---

(I suspect that #[no_start], like #[crate_id="..."], is simply totally unused at this point, and our infrastructure for warning about crate-level attributes in the wrong place in the AST is somehow mistakenly coupled to the unused_attribute analysis.)

[ppannuto]

Any headway on this issue? We'd like to enable warnings-as-errors for CI builds on Tock, but this warning is currently generated for every platform.