[flang] Update fir.dispatch operation

Update the `fir.dispatch` operation to prepare
the lowering part. `nopass` and `pass_arg_pos` attributes
are added in the arguments list so accessors are generated
by MLIR tablegen. A verifier is added as well as some tests.

This patch is part of the implementation of the poltymorphic
entities.
https://github.com/llvm/llvm-project/blob/main/flang/docs/PolymorphicEntities.md

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D135358