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

This reverts commit r313677.

Buildbots fail with assertion failure
Failing Tests (7):
    Clang :: Analysis/null-deref-ps.c
    Clang :: CodeGen/enum.c
    Clang :: Sema/compare.c
    Clang :: Sema/outof-range-constant-compare.c
    Clang :: Sema/tautological-unsigned-enum-zero-compare.c
    Clang :: Sema/tautological-unsigned-zero-compare.c
    Clang :: SemaCXX/compare.cpp

llvm-svn: 313683