Add regression test for #21562

Closes #21562.

Author: jooert

src/test/run-pass/issue-21562.rs

@@ -0,0 +1,17 @@
+#![feature(core)]
+
+extern crate core;
+use core::marker::Sync;
+
+static SARRAY: [i32; 1] = [11];
+
+struct MyStruct {
+    pub arr: *const [i32],
+}
+unsafe impl Sync for MyStruct {}
+
+static mystruct: MyStruct = MyStruct {
+    arr: &SARRAY
+};
+
+fn main() {}