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

[Nathan Chancellor via cfe-commits]

nathanchance wrote:

@vgvassilev Yes, it appears so, I tried one of the examples from [the documentation](https://clang.llvm.org/docs/ClangRepl.html) since I have no prior experience with `clang-repl`.

```
$ clang-repl --version
LLVM (http://llvm.org/):
  LLVM version 19.0.0git
  Optimized build with assertions.

$ clang-repl
clang-repl> #include <iostream>
clang-repl> int sum(int a, int b){ return a+b; };
clang-repl> int c = sum(9,10);
clang-repl> std::cout << c << std::endl;
19
clang-repl> ^D
```

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