[flang] Allow for access-stmt before namelist-stmt

You can declare a name in an access statement and then declare it as
a namelist group name. We weren't allowing that because we didn't
convert a symbol with UnknownDetails to one with NamelistDetails.

Fixes flang-compiler/f18#1022.

Original-commit: flang-compiler/f18@a759204db583d337dda886acf748e0fe135c6259
Reviewed-on: https://github.com/flang-compiler/f18/pull/1023