Don't GC DW_TAG_variant(_part), they are referenced (#2113)

by their respective parents.

See also #2105.

crates/debug/src/gc.rs

@@ -103,6 +103,8 @@ fn has_die_back_edge<R: Reader<Offset = usize>>(die: &read::DebuggingInformation
         | constants::DW_TAG_template_type_parameter
         | constants::DW_TAG_enumerator
         | constants::DW_TAG_member
+        | constants::DW_TAG_variant_part
+        | constants::DW_TAG_variant
         | constants::DW_TAG_formal_parameter => true,
         _ => false,
     }