[Clang] Fix how we set the NumPositiveBits on an EnumDecl to cover the case of single...

[Clang] Fix how we set the NumPositiveBits on an EnumDecl to cover the case of single enumerator with value zero or an empty enum

Currently in Sema::ActOnEnumBody(...) when calculating NumPositiveBits we miss
the case where there is only a single enumerator with value zero and the case of
an empty enum. In both cases we end up with zero positive bits when in fact we
need one bit to store the value zero.

This PR updates the calculation to account for these cases.

Differential Revision: https://reviews.llvm.org/D130301