Restore removed builtin_type() api method (#11932)

It shouldn't be used for new code, but adding it back makes it
unnecessary to update existing plugins that no longer worked with mypy
0.930.

Related issue: dropbox/sqlalchemy-stubs#232

Author: JukkaL

Diff for: mypy/plugin.py

@@ -257,6 +257,12 @@ def named_type(self, fullname: str,
         """Construct an instance of a builtin type with given type arguments."""
         raise NotImplementedError
 
+    @abstractmethod
+    def builtin_type(self, fully_qualified_name: str) -> Instance:
+        """Legacy function -- use named_type() instead."""
+        # NOTE: Do not delete this since many plugins may still use it.
+        raise NotImplementedError
+
     @abstractmethod
     def named_type_or_none(self, fullname: str,
                            args: Optional[List[Type]] = None) -> Optional[Instance]:

Diff for: mypy/semanal.py

@@ -4565,6 +4565,10 @@ def named_type_or_none(self, fullname: str,
             return Instance(node, args)
         return Instance(node, [AnyType(TypeOfAny.unannotated)] * len(node.defn.type_vars))
 
+    def builtin_type(self, fully_qualified_name: str) -> Instance:
+        """Legacy function -- use named_type() instead."""
+        return self.named_type(fully_qualified_name)
+
     def lookup_current_scope(self, name: str) -> Optional[SymbolTableNode]:
         if self.locals[-1] is not None:
             return self.locals[-1].get(name)