[DebugInfo] Use per-enumerator signedness for DIEnumerator

Allegedly the DWARF backend ignores this field of DIEnumerator, but we
set it nonetheless in case we decide to use it in the future.
Alternatively, we could remove it, but it is simpler to pass down the
signed bit as it is in the AST for now.

Implemented to address comments on D106585

GitOrigin-RevId: f9f56488e02d1c09a9cd4acde61ce1c712e71405
2 files changed