[flang] Accept OPEN(...,CONVERT="SWAP") in semantics (#79050)

The runtime implements CONVERT="SWAP", but semantics doesn't like it.
Add it to the relevant table.

Fixes llvm-test-suite/Fortran/gfortran/regression/record_marker_1.f90,
.../unf_io_convert_1.f90, .../unf_io_convert_2.f90, and
.../unf_io_convert_3.f90.