Ignore clang warning Wnontrivial-memaccess (#8129)

Produced when memset(this, ..) is used on a non-trivially copyable type

backends/imgui_impl_metal.mm

@@ -38,6 +38,13 @@
 #import <time.h>
 #import <Metal/Metal.h>
 
+#if defined(__clang__)
+#if __has_warning("-Wunknown-warning-option")
+#pragma clang diagnostic ignored "-Wunknown-warning-option"         // warning: unknown warning group 'xxx'
+#endif
+#pragma clang diagnostic ignored "-Wnontrivial-memaccess"           // warning: first argument in call to 'memset' is a pointer to non-trivially copyable type
+#endif
+
 #pragma mark - Support classes
 
 // A wrapper around a MTLBuffer object that knows the last time it was reused