Add a regression test for match guard

Unlike if condition, match guard accepts struct literal.

src/test/ui/parser/struct-literal-in-match-guard.rs

@@ -0,0 +1,18 @@
+// check-pass
+
+// Unlike `if` condition, `match` guards accept struct literals.
+// This is detected in <https://github.com/rust-lang/rust/pull/74566#issuecomment-663613705>.
+
+#[derive(PartialEq)]
+struct Foo {
+    x: isize,
+}
+
+fn foo(f: Foo) {
+    match () {
+        () if f == Foo { x: 42 } => {}
+        _ => {}
+    }
+}
+
+fn main() {}