[flang] Fix ParamValue attribute (kind/len) of implicit type parameters

While testing fix for issue 581 it appeared that 'ParamValue`
implicit len parameters had incorrect attribute kind.
This commit:
 + Set correct attribute when creating `ParamValue` for implicit
   type parameter.
 + Also set the correct attribute foe charachter lenght `ParamValue`
   though it is currently not used anywhere.
 + Change some std::int64_t to common::ConstantSubscript on the way.

Original-commit: flang-compiler/f18@57a344b25607903638be6df47155bfe5515fff3a
Reviewed-on: https://github.com/flang-compiler/f18/pull/615
Tree-same-pre-rewrite: false