move stage2 compile errors out of special folder

test/cases/compile_errors/union_enum_field_missing.zig

@@ -0,0 +1,21 @@
+const E = enum {
+    a,
+    b,
+    c,
+};
+
+const U = union(E) {
+    a: i32,
+    b: f64,
+};
+
+export fn entry() usize {
+    return @sizeOf(U);
+}
+
+// error
+// target=native
+//
+// :7:11: error: enum field(s) missing in union
+// :4:5: note: field 'c' missing, declared here
+// :1:11: note: enum declared here