[flang][OpenMP] atomic write on complex

[tblah]

program complex_write
  complex :: z
  !$omp atomic write
    z = cmplx(1,1,kind(x))
end program

This currently hits an assertion guarding types allowed to be used in an atomic write. Similar work has been done already for atomic update and atomic read:

• [llvm][OpenMP] Handle complex types in atomic read #111377
• [llvm][mlir][flang][OpenMP] Emit __atomic_load and __atomic_compare_exchange libcalls for complex types in atomic update #92364