Take into account the zero sign bit for positive numbers when computing the bit

Take into account the zero sign bit for positive numbers when computing the bit
width of an enum with negative values in IntRange.  Include a test for
-Wtautological-constant-out-of-range-compare where this had manifested.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@168126 91177308-0d34-0410-b5e6-96231b3b80d8