Add test for the ICE

Signed-off-by: Shunpoco <[email protected]>

Author: Shunpoco

Diff for: tests/ui/match/ref-mut-inside-ref-not-pattern-mutability-cap-violated.rs

@@ -0,0 +1,6 @@
+// This test is for https://github.com/rust-lang/rust/issues/136223.
+// `&mut` inside `&` should not panic by `Pattern mutability cap violdated` debug assertion.
+fn main() {
+  if let &Some(Some(x)) = &Some(&mut Some(0)) {}
+  //~^ ERROR: cannot borrow data in a `&` reference as mutable
+}

Diff for: tests/ui/match/ref-mut-inside-ref-not-pattern-mutability-cap-violated.stderr

@@ -0,0 +1,9 @@
+error[E0596]: cannot borrow data in a `&` reference as mutable
+  --> $DIR/ref-mut-inside-ref-not-pattern-mutability-cap-violated.rs:4:21
+   |
+LL |   if let &Some(Some(x)) = &Some(&mut Some(0)) {}
+   |                     ^ cannot borrow as mutable
+
+error: aborting due to 1 previous error
+
+For more information about this error, try `rustc --explain E0596`.