Stop using CRATE_DEF_INDEX.

`CRATE_DEF_ID` and `CrateNum::as_def_id` are almost always what we want.

compiler/rustc_span/src/def_id.rs

@@ -283,8 +283,19 @@ impl DefId {
         self.as_local().unwrap_or_else(|| panic!("DefId::expect_local: `{:?}` isn't local", self))
     }
 
+    #[inline]
+    pub fn is_crate_root(self) -> bool {
+        self.index == CRATE_DEF_INDEX
+    }
+
+    #[inline]
+    pub fn as_crate_root(self) -> Option<CrateNum> {
+        if self.is_crate_root() { Some(self.krate) } else { None }
+    }
+
+    #[inline]
     pub fn is_top_level_module(self) -> bool {
-        self.is_local() && self.index == CRATE_DEF_INDEX
+        self.is_local() && self.is_crate_root()
     }
 }
 
@@ -357,7 +368,7 @@ impl LocalDefId {
 
     #[inline]
     pub fn is_top_level_module(self) -> bool {
-        self.local_def_index == CRATE_DEF_INDEX
+        self == CRATE_DEF_ID
     }
 }