[mlir][IR] SingleBlockImplicitTerminator: Declare "inherited" trait in ODS instead of C++

`SingleBlockImplicitTerminator` is now a combination of two traits: `SingleBlock` and `SingleBlockImplicitTerminatorImpl` (the original `SingleBlockImplicitTerminator`).

This change makes it possible to check if the `SingleBlock` op trait is implemented. Until now, `Operation::hasTrait<OpTrait::SingleBlock>()` returned `false` for ops that implement `SingleBlockImplicitTerminator`.

Differential Revision: https://reviews.llvm.org/D159078