-Wunused-private-field false negative with template instantiation

[AaronBallman]

Consider code like:

template <typename Ty, typename Uy>
class C2 {
  Ty c; // no-warning
  Uy d; // warning
};

class [[maybe_unused]] Good {};
class Bad {};

C2<Good, Bad> c;

Clang fails to diagnose d: https://godbolt.org/z/qjE5sxqff

This was discovered during review of https://reviews.llvm.org/D159083, for more context.

[llvmbot]

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