Clang-Tidy: Ignore modernize-use-designated-initializers lint

This lint, which warns about not using designated initializers for
aggregate types initialization, became active with the C++20 upgrade.

Currently, it causes *a lot* of warnings, especially in places where
this wouldn't be desirable, such as RNA enum declarations. Add it to the
list of silenced modernize- lints as such.

Pull Request: https://projects.blender.org/blender/blender/pulls/154613

.clang-tidy

@@ -55,6 +55,7 @@ Checks:  >
   -modernize-pass-by-value,
   -modernize-raw-string-literal,
   -modernize-return-braced-init-list,
+  -modernize-use-designated-initializers,
 
   objc-*