[flang] Create fir.dispatch_table and fir.dt_entry operations

Create the fir.dispatch_table operation based on semantics
information. The fir.dispatch_table will be used for static devirtualization
as well as for fir.select_type conversion.

Depends on D138129

Reviewed By: jeanPerier, PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D138131