[clang] [Clang][Sema] Reject undeduced static data members without initializers (#173349) (PR #174281)

[Yanzuo Liu via cfe-commits]

https://github.com/zwuis commented:

While we are here, could you handle the case that `u` has initializer?

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> So, should I leave it as `Type->isUndeducedType()`  and find another condition to force calling `Sema::DeduceVariableDeclarationType`  (so as to call `deduceVarTypeFromInitializer` to hit the place where we handle `E`, `U`)

I'm not familiar enough with this part of code. Please wait for other reviewers.

https://github.com/llvm/llvm-project/pull/174281