[flang] Support lowering of intrinsic module procedure `c_loc`

As Fortran 2018 18.2.3.6, the intrinsic `c_loc(x)` gets the C address
of argument `x`. It returns the scalar of type C_PTR. As defined in
iso_c_binding in `flang/module/__fortran_builtins.f90`, C_PTR is the
derived type with only one component of integer 64.

This supports the lowering of intrinsic module procedure `c_loc` by
converting the address of argument into integer 64, where the argument
is lowered as Box and the address is generated using fir.box_addr.

The lowering of intrinsic `c_funloc` has the similar characteristic and
will be supported later.

The execution tests for various data types are in issue
https://github.com/llvm/llvm-project/issues/56552.

Reviewed By: Jean Perier

Differential Revision: https://reviews.llvm.org/D129659