[mlir][ODS] Refactor BuildableType to use $_builder as part of the format

Summary:
Currently BuildableType is assumed to be preceded by a builder. This prevents constructing types that don't have a callable 'get' method with the builder. This revision reworks the format to be like attribute builders, i.e. by accepting $_builder within the format itself.

Differential Revision: https://reviews.llvm.org/D73736