[flang] Support intrinsic `selected_int_kind` for variables

As Fortran 2018 16.9.169, the argument of selected_int_kind is integer
scalar, and result is default integer scalar. The constant expression in
this intrinsic has been supported by folding the constant expression.
This supports lowering and runtime for variables in this intrinsic.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D129959