[flang][OpenMP] Pass semantics context to all generating functions in… (#81269)

… lower

The convention is to pass it after "symTable" if present, otherwise
after "converter":
- converter, symTable, semaCtx
- converter, semaCtx

This makes the interfaces more uniform---some of these functions were
already taking the semantics context, while others were not.

The context will be used in future patches.