Rollup merge of #138033 - obi1kenobi:pg/json-attrs-tests, r=aDotInTheVoid

rustdoc: Add attribute-related tests for rustdoc JSON.

Add rustdoc JSON tests covering the use of the following attributes:
- `#[non_exhaustive]` applied to enums, variants, and structs
- `#[must_use]`, both with and without a message
- `#[no_mangle]`, in both edition 2021 and 2024 (`#[unsafe(no_mangle)]`) flavors
- `#[export_name]`, also in both edition 2021 and 2024 flavors

Related to #137645; this is a subset of the attributes that `cargo-semver-checks` relies on and tests in its own test suite or in the test suites of its components such as `trustfall-rustdoc-adapter`.

Helps with #81359

r? `@aDotInTheVoid`

Author: jhpratt