[clang] [clang-repl] Teach clang-repl how to load PCHs (reprise) (PR #157359)

[Jeaye Wilkerson via cfe-commits]

jeaye wrote:

I've scoured the latest Linux/Windows build+test logs and the failures in there have seemingly nothing to do with this PR. Based on comparing these results with other recently opened PRs, it's unclear to me if these are consistent. @vgvassilev do you know?

https://github.com/llvm/llvm-project/pull/157359