use base desc for temporary base types
use base desc for temporary base types

Summary: Closer to how NumT / StrT / BoolT are made, and how errors on them are reported.

Reviewed By: panagosg7

Differential Revision: D13254166

fbshipit-source-id: 537494cbd04375ee581d19884abf782cf1d8c952

nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow,nmote/flow