[flang] Add support for lowering intrinsic module procedure C_ASSOCIATED

As Fortran 2018 18.2.3.2, the arguments of C_ASSOCIATED have the same
type, a scalar of type C_PTR or C_FUNPTR, and the result has the default
logical scalar type. The interface is defined with two module procedures
c_associated_c_ptr/c_associated_c_funptr in flang/module/iso_c_binding.
The result is false if the first argument is a C null pointer. If the
second argument is present, the result is true only if the two arguments
are equal. Support the lowering by comparing the C pointer address
values of two arguments if the second argument is dynamically present
and comparing the C pointer address value of the first argument with the
value 0.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136419