[mlir] Use TypedValue in single result traits

Ops with a single result currently get a `getResult()` method +
conversion operator to `Value` through the `OneResult` trait. By moving
these to the `OneTypedResult` trait instead, we can use `TypedValue` as
the return type to get more specfic types.

When the result type is unknown ODS adds the
`OneTypedResult<mlir::Type>` trait, in which case there is no change in
the resulting API.

Reviewed By: rriddle

Differential Revision: https://reviews.llvm.org/D142852