Rollup merge of rust-lang#66786 - jyn514:const-if-match-tests, r=Centril

Add wildcard test for const_if_match

Closes rust-lang#66758

Many thanks to @Centril for his help getting me started!

src/test/ui/consts/control-flow/single-arm-match-wild.rs

@@ -0,0 +1,21 @@
+// check-pass
+
+#![feature(const_if_match)]
+
+enum E {
+    A,
+    B,
+    C
+}
+
+const fn f(e: E) -> usize {
+    match e {
+        _ => 0
+    }
+}
+
+fn main() {
+    const X: usize = f(E::C);
+    assert_eq!(X, 0);
+    assert_eq!(f(E::A), 0);
+}