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

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

For what it's worth, this test appears to fail when `LLVM_DEFAULT_TARGET_TRIPLE` is changed (my script sets it to the output of my Linux distribution's `clang -print-target-triple`)

```
error: PCH file was compiled for the target 'x86_64-pc-linux-gnu' but the current translation unit is being compiled for target 'x86_64-unknown-linux-gnu'
error: module file .../tools/clang/test/Interpreter/Output/cxx20-modules.cppm.tmp/mod.pcm cannot be loaded due to a configuration mismatch with the current compilation [-Wmodule-file-config-mismatch]
error: Parsing failed.
```

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