Identifier: Preserve invalid 32-bit values.

When we initialize an Identifier from a 32-bit value, make sure
to preserve invalid values.  We were just shifting left by 32 bits,
which means that the invalid 0xffffffff becomes 0xffffffff00000000,
for which is_valid() returns true.  This was causing us not to detect
some invalid identifiers after they were saved and reloaded.