Introduce minimal Refaster support for [unnamed variables](https://openjdk.org/jeps/456).

There is probably more to be worked out here, even to decide upon the full set of desired behaviors. But it's nice to at least not crash.

PiperOrigin-RevId: 940686822

Author: cpovirk

core/src/main/java/com/google/errorprone/refaster/UVariableDecl.java

@@ -131,6 +131,9 @@ private JCVariableDecl inline(@Nullable UExpression type, Inliner inliner)
       modifiers = maker.Modifiers(0L);
       name = getName().inline(inliner);
     }
+    if (name.isEmpty()) {
+      name = inliner.asName("_");
+    }
     return maker.VarDef(
         modifiers,
         name,