Disallows `#![feature(no_coverage)]` on stable and beta (using standard crate-level gating)
Fixes: #84836
Removes the function-level feature gating solution originally implemented, and solves the same problem using `allow_internal_unstable`, so normal crate-level feature gating mechanism can still be used (which disallows the feature on stable and beta).
I tested this, building the compiler with and without `CFG_DISABLE_UNSTABLE_FEATURES=1`
With unstable features disabled, I get the expected result as shown here:
```shell
$ ./build/x86_64-unknown-linux-gnu/stage1/bin/rustc src/test/run-make-fulldeps/coverage/no_cov_crate.rs
error[E0554]: `#![feature]` may not be used on the dev release channel
--> src/test/run-make-fulldeps/coverage/no_cov_crate.rs:2:1
|
2 | #![feature(no_coverage)]
| ^^^^^^^^^^^^^^^^^^^^^^^^
error: aborting due to previous error
For more information about this error, try `rustc --explain E0554`.
```
r? ````@Mark-Simulacrum````
cc: ````@tmandry```` ````@wesleywiser````
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