structural_match: non-structural-match ty closures

This commit adds a `Closure` variant to `NonStructuralMatchTy` in
`structural_match`, fixing an ICE which can occur when
`impl_trait_in_bindings` is used with constants.

Signed-off-by: David Wood <david@davidtw.co>

src/test/ui/impl-trait-in-bindings-issue-73003.rs

@@ -0,0 +1,8 @@
+// check-pass
+
+#![feature(impl_trait_in_bindings)]
+//~^ WARN the feature `impl_trait_in_bindings` is incomplete
+
+const _: impl Fn() = ||();
+
+fn main() {}

src/test/ui/impl-trait-in-bindings-issue-73003.stderr

@@ -0,0 +1,11 @@
+warning: the feature `impl_trait_in_bindings` is incomplete and may not be safe to use and/or cause compiler crashes
+  --> $DIR/impl-trait-in-bindings-issue-73003.rs:3:12
+   |
+LL | #![feature(impl_trait_in_bindings)]
+   |            ^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `#[warn(incomplete_features)]` on by default
+   = note: see issue #63065 <https://github.com/rust-lang/rust/issues/63065> for more information
+
+warning: 1 warning emitted
+