incremental: add new test case

This isn't exactly the case provided in #11290, but is a slightly
simpler case which I know would have triggered the same bug in the old
implementation of incremental compilation.

Resolves: #11290

test/incremental/remove_invalid_union_backing_enum

@@ -0,0 +1,30 @@
+#target=x86_64-linux-selfhosted
+#target=x86_64-linux-cbe
+#target=x86_64-windows-cbe
+#update=initial version
+#file=main.zig
+const E = enum { a, b, c };
+const U = union(E) {
+    a: i32,
+    b: f64,
+    c: f64,
+    d: f64,
+};
+pub fn main() void {
+    const u: U = .{ .a = 123 };
+    _ = u;
+}
+#expect_error=ignored
+#update=remove invalid backing enum
+#file=main.zig
+const U = union {
+    a: i32,
+    b: f64,
+    c: f64,
+    d: f64,
+};
+pub fn main() void {
+    const u: U = .{ .a = 123 };
+    _ = u;
+}
+#expect_stdout=""