[clang] [Sema] Warn about omitting deprecated enumerator in switch (PR #138562)

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ReticulateSplines wrote:

Fly-by comment: I drafted the following alternative comment in another thread. Please feel free to disregard as I don't know the tone and style of comments in these files. (But I appreciate your consideration for the contents in the message I'd like to see!)

          // We currently treat deprecated constants as if they don't exist;
          // however, note that this current behavior leads to compile-time
          // false negatives for coverage checking of switch statements. If
          // a switch is missing a case for a deprecated constant, we will not
          // emit a diagnostic, even though the deprecated constant might still
          // be present in legacy use.

https://github.com/llvm/llvm-project/pull/138562