Fixes missing overflow lint for i64 #14269

The `type_overflow` lint, doesn't catch the overflow for `i64` because
the overflow happens earlier in the parse phase when the `u64` as biggest
possible int gets casted to `i64` , without checking the for overflows.
We can't lint in the parse phase, so a refactoring of the `LitInt` type
was necessary.

The types `LitInt`, `LitUint` and `LitIntUnsuffixed` where merged to one
type `LitInt` which stores it's value as `u64`. An additional parameter was
added which indicate the signedness of the type and the sign of the value.

src/libsyntax/ext/deriving/generic/mod.rs

@@ -996,7 +996,7 @@ impl<'a> MethodDef<'a> {
             let arms : Vec<ast::Arm> = variants.iter().enumerate()
                 .map(|(index, &variant)| {
                     let pat = variant_to_pat(cx, sp, &*variant);
-                    let lit = ast::LitUint(index as u64, ast::TyU);
+                    let lit = ast::LitInt(index as u64, ast::UnsignedIntLit(ast::TyU));
                     cx.arm(sp, vec![pat], cx.expr_lit(sp, lit))
                 }).collect();