add a function for testing if a ty is uninhabited from all modules

Desired by miri.

src/librustc/ty/inhabitedness/mod.rs

@@ -108,6 +108,10 @@ pub fn is_ty_uninhabited_from(self, module: DefId, ty: Ty<'tcx>) -> bool {
         self.ty_inhabitedness_forest(ty).contains(self, module)
     }
 
+    pub fn is_ty_uninhabited_from_all_modules(self, ty: Ty<'tcx>) -> bool {
+        !self.ty_inhabitedness_forest(ty).is_empty()
+    }
+
     fn ty_inhabitedness_forest(self, ty: Ty<'tcx>) -> DefIdForest {
         ty.uninhabited_from(&mut FxHashMap(), self)
     }