[flang] Answer comments

- make `TypaParamAttr` a `ParamValue` argument. Modify
  `GetParamValue` to also take this as an argument.
- remove `GetLenParamValue` that is now useless and
  modify constructor/`GetParamValue` calls
- get it the `TypeParamAttr` right from the begining
  when visiting `parser::DerivedTypeSpec` in resolve-names.cc.
  It was set to `Kind` by default and it was hard to ensure the
  attribute would not be checked until set properly.

Original-commit: flang-compiler/f18@bcc300e7141db6f692579e1327557c9831b9c85a
Reviewed-on: https://github.com/flang-compiler/f18/pull/615