syntax: Fix printing INT64_MIN

Integers are always parsed as a u64 in libsyntax, but they're stored as i64. The
parser and pretty printer both printed an i64 instead of u64, sometimes
introducing an extra negative sign.

src/test/run-pass/big-literals.rs

@@ -16,4 +16,5 @@ pub fn main() {
 
     assert_eq!(-2147483648i32 - 1i32, 2147483647i32);
     assert_eq!(-9223372036854775808i64 - 1i64, 9223372036854775807i64);
+    assert_eq!(-9223372036854775808 - 1, 9223372036854775807);
 }