unreachable!() reached when trying to assign a void call result to a pointer

[mhasel]

When trying to run the following code, the compiler hits an unreachable in llvm_typesystem::cast_if_needed without a validation error being reported:

FUNCTION_BLOCK fb_t
VAR_INPUT
    x : DINT;
END_VAR
END_FUNCTION_BLOCK

FUNCTION main : DINT
VAR
    fb : fb_t;
    y : STRING := 'hello';
END_VAR
    fb := fb(1); // panics
    y := fb(1); // panics
END_FUNCTION

We should add a validation to catch assignments to pointers, where the RHS is void.