Add #[inline] to some new functions

Co-authored-by: Camille Gillot <gillot.camille@gmail.com>

compiler/rustc_span/src/def_id.rs

@@ -186,6 +186,7 @@ impl StableCrateId {
         StableCrateId(hasher.finish())
     }
 
+    #[inline]
     pub fn as_u64(self) -> u64 {
         self.0.as_u64()
     }