[flang] automatically load FIR dialect with hlfir

MLIR loads dialects lazily so if a hlfir type (or operation) is found
before any fir type (or operation), the fir dialect will not have been
loaded when the hlfir thing is verified. Verification of some hlfir
operations does depend on fir types (e.g. hlfir.sum needs
fir::SequenceType).

Tablegen change recommended by jeanPerier

Differential Revision: https://reviews.llvm.org/D143930