Arena-allocate hir::Lifetime.

This shrinks `hir::Ty` from 72 to 48 bytes.

`visit_lifetime` is added to the HIR stats collector because these types
are now stored in memory on their own, instead of being within other
types.

Author: nnethercote

clippy_utils/src/hir_utils.rs

@@ -929,7 +929,7 @@ impl<'a, 'tcx> SpanlessHash<'a, 'tcx> {
         }
     }
 
-    pub fn hash_lifetime(&mut self, lifetime: Lifetime) {
+    pub fn hash_lifetime(&mut self, lifetime: &Lifetime) {
         std::mem::discriminant(&lifetime.name).hash(&mut self.s);
         if let LifetimeName::Param(param_id, ref name) = lifetime.name {
             std::mem::discriminant(name).hash(&mut self.s);