[clang] [Clang] [Sema] Don't crash on unexpanded pack in invalid block literal (PR #110762)

[John McCall via cfe-commits]

rjmccall wrote:

I don't think there's a deep reason blocks and lambdas don't use quite the same scope mechanics in the compiler, so if you wanted to pursue that, it seems reasonable.  But this approach also seems viable.  I agree that removing the assertion would be the wrong thing to do, and I also don't see a reliable way to include the pack in the AST.

https://github.com/llvm/llvm-project/pull/110762