[NFC][flang][OpenMP] Unify `genSectionsOp`'s prototype to match other `genXXXOp` functions (#144013)
Unifies the prototype of `genSectionsOp` to match other ops generators.
Doing so, we are able to call `genSectionsOp` directtly from
`genOMPDispatch` instead of the special handling needed now to pass the
section blocks. This is useful because now we can handle symbol mapping
scopes easier for nested OpenMP directives. See
https://github.com/llvm/llvm-project/pull/143706#issuecomment-2965344723
and the following discussion for more info.