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

[via cfe-commits]

Sirraide wrote:

> I can instead look into refactoring blocks to work the same way as lambdas

Surprisingly, this turned out to be fairly straight-forward to implement. 

(I had to fix something in the template instantiator, but I’ve run into the exact same problem before with lambdas, so I knew what to do this time round ;Þ)

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