[clang] Support C++20 Modules in clang-repl (PR #79261)

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

This is out of my wheelhouse to debug to be honest but it seems to me that `clang-repl` does not respect `LLVM_DEFAULT_TARGET_TRIPLE` somehow? This test does the same thing as other tests to build modules but this appears to be the only one that uses `clang-repl` to consume the `.pcm`.

https://github.com/llvm/llvm-project/pull/79261