Add test for rust-lang#60407.

Author: Centril

src/test/ui/existential_types/issue-60407.rs

@@ -0,0 +1,15 @@
+// check-pass
+
+#![feature(existential_type)]
+
+existential type Debuggable: core::fmt::Debug;
+
+static mut TEST: Option<Debuggable> = None;
+
+fn main() {
+    unsafe { TEST = Some(foo()) }
+}
+
+fn foo() -> Debuggable {
+    0u32
+}