Don't silently truncate integer constants

[sim642]

While looking into goblint/analyzer#1794, I was surprised to find that CIL silently truncates too large integer constants on attribute-less types:

int main( void ) {
  volatile int a = 121111111111111; // Goblint sees (int volatile)121111111111111
  int b = 121111111111111; // Goblint just sees 1623298503
  return 0;
}

After lots of digging, this seems to be done here by kintegerCilint:

cil/src/cil.ml

Lines 6100 to 6113 in a35d59e

	let mkCastT ~(e: exp) ~(oldt: typ) ~(newt: typ) =
	(* Do not remove old casts because they are conversions !!! *)
	if Util.equals (typeSig oldt) (typeSig newt) then begin
	e
	end else begin
	(* Watch out for constants *)
	match newt, e with
	(* Casts to _Bool are special: they behave like "!= 0" ISO C99 6.3.1.2 *)
	TInt(IBool, []), Const(CInt(i, _, _)) ->
	let v = if compare i zero_cilint = 0 then zero_cilint else one_cilint in
	Const (CInt(v, IBool, None))
	| TInt(newik, []), Const(CInt(i, _, _)) -> kintegerCilint newik i
	| _ -> CastE(newt,e)
	end

It seems weird, especially to do it silently while arbitrarily depending on the presence of attributes. It prevents Goblint from soundly reporting overflows on constants.