Resolves#15335: getSymbolsByUDA fails if type has private members.
Generous (ab)use of mixins allows getSymbolsByUDA to reference symbols
without trying to access them, allowing it to inspect private members
without failing.
Testing this required private symbols defined outside of std.traits.
This adds std.internal.test.uda, which defines a struct containing
private members that is used in a unittest added to std.traits.
