implement packed struct equality (#21679)

test/cases/compile_errors/packed_struct_comparison.zig

@@ -0,0 +1,35 @@
+const x: Foo = .{};
+const y: Foo = .{};
+
+export fn a() void {
+    _ = x > y;
+}
+
+export fn b() void {
+    _ = x < y;
+}
+
+export fn c() void {
+    _ = x >= y;
+}
+export fn d() void {
+    _ = x <= y;
+}
+
+const Foo = packed struct {
+    a: u4 = 10,
+    b: u4 = 5,
+};
+
+// error
+// backend=stage2
+// target=native
+//
+// :5:11: error: operator > not allowed for type 'tmp.Foo'
+// :19:20: note: struct declared here
+// :9:11: error: operator < not allowed for type 'tmp.Foo'
+// :19:20: note: struct declared here
+// :13:11: error: operator >= not allowed for type 'tmp.Foo'
+// :19:20: note: struct declared here
+// :16:11: error: operator <= not allowed for type 'tmp.Foo'
+// :19:20: note: struct declared here