[Sema] Move 'char-expression-as-unsigned < 0' into a separate diagnostic

This change splits '-Wtautological-unsigned-zero-compare' by reporting
char-expressions-interpreted-as-unsigned under a separate diagnostic
'-Wtautological-unsigned-char-zero-compare'. This is beneficial for
projects that want to enable '-Wtautological-unsigned-zero-compare' but at
the same time want to keep code portable for platforms with char being
signed or unsigned, such as Chromium.

Differential Revision: https://reviews.llvm.org/D99808