[Sema] Put tautological comparison of unsigned and zero into it's own flag

Summary:
As requested by Sam McCall.

```
$ /build/llvm-build-Clang-release/./bin/clang -c /build/clang/test/Sema/outof-range-constant-compare.c /build/clang/test/Sema/outof-range-constant-compare.c:40:11: warning: comparison of unsigned expression < 0 is always false [-Wtautological-unsigned-zero-compare]
    if (a < 0x0000000000000000UL) // expected-warning {{comparison of unsigned expression < 0 is always false}}
        ~ ^ ~~~~~~~~~~~~~~~~~~~~
```

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

Reviewed By: sammccall, aaron.ballman

Subscribers: aaron.ballman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D37620

llvm-svn: 312792