[clang] Sema: filter out invalid base-specifiers before attaching (PR #147213)

[Corentin Jabot via cfe-commits]

https://github.com/cor3ntin commented:

Thanks for working on this

Did you try  to use `!isUsable` instead of IsInvalid in
`ParseBaseSpecifier` ? I think that might be a simpler fix.

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https://github.com/llvm/llvm-project/pull/147213