[clang] [clang] fix error recovery for invalid member specializations (PR #207068)

[Matheus Izvekov via cfe-commits]

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mizvekov wrote:

I don't think we have to commit to one or another. Some problems are easy to recover without throwing away the whole declaration.

For example, it's easy to recover from an invalid type used in the declaration.

But recovering from a member specialization of a member that doesn't exist is cumbersome without throwing away the declaration, as we would otherwise have to go back and create a fake member for a template which has already been instantiated. Doable but doesn't seem worth it IMO.

https://github.com/llvm/llvm-project/pull/207068