[flang] Fix writing defined operators to .mod files

When defined operators were written to .mod files in USE statement
they did not come out correctly. They have to be emitted with
`PutGenericName()` so that `operator` is included.

Original-commit: flang-compiler/f18@d40e65a2f9c176f5334adb8132ef65273cf3615f
Reviewed-on: https://github.com/flang-compiler/f18/pull/675
Tree-same-pre-rewrite: false