[flang] Resolve namelist group and object names

Namelist groups are represents by symbols with `NamelistDetails`.
Those contain a list of symbols representing the objects in the group.
References to namelist groups in io-control-spec are resolved.

In `.mod` files, namelist groups are written out at the end of the
module specification section. This is so that the declarations of the
objects in the namelist group appear before they are referenced.

Original-commit: flang-compiler/f18@8b70dbcac77f287725a8772899c5ea5ff093e25f
Reviewed-on: https://github.com/flang-compiler/f18/pull/277
Tree-same-pre-rewrite: false