Incorrect check of maximum literal value

[treiher]

Ada only allows literal values up to 2^63 - 1, even if the size of the enum is 64 bit.

package Test is

    type A is (B => 9223372036854775808) with Size => 64;  --  2**63 = 9223372036854775808

end Test;

rflx-test.ads:20:20: error: value outside permitted range

   compilation of rflx-test.ads failed

 17    type A is (B) with
 18      Size =>
 19        64;
 20    for A use (B => 9223372036854775808);