Refactor: ASR: Handle c_p_pointer() in visit_Call()

[Shaikh-Ubaid]

This PR removes the use of

is_c_pointer_call_copy = is_c_pointer_call;
visit_expr(...);
if (is_c_pointer_call) {
    ...
}
is_c_pointer_call = is_c_pointer_call_copy;

and handles the c_p_pointer() in visit_Call(),

thus making the code clean/tidy.

[Shaikh-Ubaid]

The c_p_pointer() needs the variable to which it is being assigned. Therefore, in this PR, we first add the variable to scope, then visit the value/c_p_pointer() and later set the variable's initial value (init_val) in process_variable_init_val().

[certik]

I think this is fine, thanks!