switch to an allowlist approach

- merge error codes
- use attribute name that is incompatible in error message
- add test for conditional incompatible attribute
- add `linkage` to the allowlist

compiler/rustc_error_codes/src/error_codes/E0736.md

@@ -1,11 +1,12 @@
 Functions marked with the `#[naked]` attribute are restricted in what other
-code generation attributes they may be marked with.
+attributes they may be marked with.
 
-The following code generation attributes are incompatible with `#[naked]`:
+Notable attributes that are incompatible with `#[naked]` are:
 
-  * `#[inline]`
-  * `#[track_caller]`
-  * `#[target_feature]`
+* `#[inline]`
+* `#[track_caller]`
+* `#[target_feature]`
+* `#[test]`, `#[ignore]`, `#[should_panic]`
 
 Erroneous code example:
 
@@ -18,7 +19,3 @@ fn foo() {}
 These incompatibilities are due to the fact that naked functions deliberately
 impose strict restrictions regarding the code that the compiler is
 allowed to produce for this function.
-
-See [the reference page for codegen attributes] for more information.
-
-[the reference page for codegen attributes]: https://doc.rust-lang.org/reference/attributes/codegen.html

compiler/rustc_error_codes/src/error_codes/E0798.md

@@ -1,14 +0,0 @@
-Testing attributes cannot be applied to functions marked with `#[naked]`.
-
-Erroneous code example:
-
-```ignore (requires test runner)
-#[test]
-#[should_panic]
-#[naked]
-fn foo() {}
-```
-
-See [the reference page for testing attributes] for more information.
-
-[the reference page for testing attributes]: https://doc.rust-lang.org/reference/attributes/testing.html