[flang] Support lowering intrinsic `selected_real_kind` for variables

As Fortran 2018 16.9.170, the argument of `selected_real_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 this intrinsic for variables using runtime.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D130183