stage2: add error for non-void error union payload being ignored

See ziglang#6060 (comment)

compile_errors/non_void_error_union_payload_ignored.zig

@@ -0,0 +1,25 @@
+pub export fn entry1() void {
+    var x: anyerror!usize = 5;
+    if (x) {
+        // foo
+    } else |_| {
+        // bar
+    }
+}
+pub export fn entry2() void {
+    var x: anyerror!usize = 5;
+    while (x) {
+        // foo
+    } else |_| {
+        // bar
+    }
+}
+
+// error
+// backend=stage2
+// target=native
+//
+// :3:5: error: error union payload is ignored
+// :3:5: note: payload value can be explicitly ignored with '|_|'
+// :11:5: error: error union payload is ignored
+// :11:5: note: payload value can be explicitly ignored with '|_|'