Generate warnings if a downcast is involved in a function call

[volsa]

Given the following code snippet

FUNCTION fn_int : INT
    VAR_INPUT {ref}
        in_ref : INT;
    END_VAR
    VAR_IN_OUT
        in_out : INT;
    END_VAR
END_FUNCTION

PROGRAM main
    VAR
        var1_lint, var2_lint : LINT := 4;
    END_VAR

    fn_int(var1_lint, var2_lint);
END_PROGRAM

Rusty currently does not generate any warnings although the arguments are incompatible with the function signature. Once issue #698 is implemented, whether it's going to be in the form of a bitcast[1] or a truncate instruction, potential data-loss might be involved. Thus it would be nice if some warnings are generated for the end-user. For reference, clang generates something along

warning: incompatible pointer types passing 'long *' to parameter of type 'int *' [-Wincompatible-pointer-types]

---

[1] as it is implemented in the current PR where the IR looks similar to its C counter-part, i.e.

  %var1_lint = getelementptr inbounds %main, %main* %0, i32 0, i32 0
  %var2_lint = getelementptr inbounds %main, %main* %0, i32 0, i32 1
  %1 = bitcast i64* %var1_lint to i16*
  %2 = bitcast i64* %var2_lint to i16*
  %call = call i16 @fn_int(i16* %1, i16* %2)