[Sema] Move some stuff into -Wtautological-unsigned-enum-zero-compare

Summary:
As requested by Sam McCall:
> Enums (not new I guess). Typical case: if (enum < 0 || enum > MAX)
> The warning strongly suggests that the enum < 0 check has no effect
> (for enums with nonnegative ranges).
> Clang doesn't seem to optimize such checks out though, and they seem
> likely to catch bugs in some cases. Yes, only if there's UB elsewhere,
> but I assume not optimizing out these checks indicates a deliberate
> decision to stay somewhat compatible with a technically-incorrect
> mental model.
> If this is the case, should we move these to a
> -Wtautological-compare-enum subcategory?

Reviewers: rjmccall, rsmith, aaron.ballman, sammccall, bkramer, djasper

Reviewed By: aaron.ballman

Subscribers: jroelofs, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D37629

llvm-svn: 313677