Add and use stability helper methods

This avoids an ambiguity (when reading) where `.level.is_stable()` is
not immediately clear whether it is general stability or const
stability.

compiler/rustc_attr/src/builtin.rs

@@ -101,6 +101,16 @@ pub struct Stability {
     pub feature: Symbol,
 }
 
+impl Stability {
+    pub fn is_unstable(&self) -> bool {
+        self.level.is_unstable()
+    }
+
+    pub fn is_stable(&self) -> bool {
+        self.level.is_stable()
+    }
+}
+
 /// Represents the `#[rustc_const_unstable]` and `#[rustc_const_stable]` attributes.
 #[derive(Encodable, Decodable, Copy, Clone, Debug, PartialEq, Eq, Hash)]
 #[derive(HashStable_Generic)]
@@ -111,6 +121,16 @@ pub struct ConstStability {
     pub promotable: bool,
 }
 
+impl ConstStability {
+    pub fn is_const_unstable(&self) -> bool {
+        self.level.is_unstable()
+    }
+
+    pub fn is_const_stable(&self) -> bool {
+        self.level.is_stable()
+    }
+}
+
 /// The available stability levels.
 #[derive(Encodable, Decodable, PartialEq, Copy, Clone, Debug, Eq, Hash)]
 #[derive(HashStable_Generic)]