ART: DCHECK zero case for CLZ/CTZ

Add a DCHECK_CONSTEXPR. All current callers have an explicit
zero check before hand and so we should not trip this at the
moment. Remove the TODO.

Add the check for T being unsigned for CLZ (trivial fix for
leb128.h). We use CTZ with signed types.

Change-Id: I7bbf0b1699eed21715c6cc20dbfe22b7da403b1a