Prevent forbid from being ignored if overriden at the same level.

That is, this changes `#[forbid(foo)] #[allow(foo)]` from allowing foo to
forbidding foo.

tests/ui/attrs.rs

@@ -3,7 +3,6 @@
 // Test that the whole restriction group is not enabled
 #![warn(clippy::restriction)]
 #![deny(clippy::restriction)]
-#![forbid(clippy::restriction)]
 #![allow(clippy::missing_docs_in_private_items, clippy::panic, clippy::unreachable)]
 
 #[inline(always)]

tests/ui/attrs.stderr

@@ -1,5 +1,5 @@
 error: you have declared `#[inline(always)]` on `test_attr_lint`. This is usually a bad idea
-  --> $DIR/attrs.rs:9:1
+  --> $DIR/attrs.rs:8:1
    |
 LL | #[inline(always)]
    | ^^^^^^^^^^^^^^^^^
@@ -7,7 +7,7 @@ LL | #[inline(always)]
    = note: `-D clippy::inline-always` implied by `-D warnings`
 
 error: the since field must contain a semver-compliant version
-  --> $DIR/attrs.rs:29:14
+  --> $DIR/attrs.rs:28:14
    |
 LL | #[deprecated(since = "forever")]
    |              ^^^^^^^^^^^^^^^^^
@@ -15,7 +15,7 @@ LL | #[deprecated(since = "forever")]
    = note: `-D clippy::deprecated-semver` implied by `-D warnings`
 
 error: the since field must contain a semver-compliant version
-  --> $DIR/attrs.rs:32:14
+  --> $DIR/attrs.rs:31:14
    |
 LL | #[deprecated(since = "1")]
    |              ^^^^^^^^^^^
@@ -37,13 +37,5 @@ LL | #![deny(clippy::restriction)]
    |
    = help: try enabling only the lints you really need
 
-error: restriction lints are not meant to be all enabled
-  --> $DIR/attrs.rs:6:11
-   |
-LL | #![forbid(clippy::restriction)]
-   |           ^^^^^^^^^^^^^^^^^^^
-   |
-   = help: try enabling only the lints you really need
-
-error: aborting due to 6 previous errors
+error: aborting due to 5 previous errors