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

[Nikita Popov via cfe-commits]

nikic wrote:

Normal clang creates a driver using the default target triple here: https://github.com/llvm/llvm-project/blob/754a8add57098ef71e4a51a9caa0cc175e94377d/clang/tools/driver/driver.cpp#L485

clang-repl appears to use IncrementalCompilerBuilder, which uses `getProcessTriple()` here: https://github.com/llvm/llvm-project/blob/754a8add57098ef71e4a51a9caa0cc175e94377d/clang/lib/Interpreter/Interpreter.cpp#L165-L166

getProcessTriple() is based on LLVM_HOST_TRIPLE with some fixups.

I'm not really sure what the right fix here is. It makes sense that an interpreter uses the host triple by default, but it's also a problem that clang and clang-repl use different default triples.

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