Promote maybe undefined into a warning

Summary: Seems that people do care and want this to show up.

Reviewed By: themarwhal

Differential Revision: D47150378

fbshipit-source-id: af95bdcf389998d0a92bf05e8ea979491ee841e3

Author: ndmitchell

starlark/src/analysis/names.rs

@@ -69,7 +69,7 @@ pub(crate) enum NameWarning {
 impl LintWarning for NameWarning {
     fn severity(&self) -> EvalSeverity {
         match self {
-            Self::UsingUnassigned(..) => EvalSeverity::Warning,
+            Self::UsingUnassigned(..) | Self::UsingMaybeUndefined(..) => EvalSeverity::Warning,
             _ => EvalSeverity::Disabled,
         }
     }