[clang] [clang-repl] Keep implicit instantiations of a failed input (PR #226253)

[Emery Conrad via cfe-commits]

conrade-ctc wrote:

> That sounds like the right concept; I don't have time today to think through how that would work.

I've got an update, I'll post shortly... it's orthogonal to the other issues above (which I also have a solution to locally), and if they are truly orthogonal, I'll get the other up in a PR and mark it as paired with this one (or we could just put the two together here, but I think they are truly orthogonal so maybe deserve separate treatments).

https://github.com/llvm/llvm-project/pull/226253