coverage: Forbid multiple #[coverage(..)] attributes

It might make sense to allow this in the future, if we add values that aren't
mutually exclusive, but for now having multiple coverage attributes on one item
is useless.

compiler/rustc_feature/src/builtin_attrs.rs

@@ -479,7 +479,7 @@ pub const BUILTIN_ATTRIBUTES: &[BuiltinAttribute] = &[
     ),
     gated!(
         coverage, Normal, template!(Word, List: "on|off"),
-        WarnFollowing, EncodeCrossCrate::No,
+        ErrorPreceding, EncodeCrossCrate::No,
         coverage_attribute, experimental!(coverage)
     ),