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

This comes from when I playing around clang-repl with moduels : )

I succeeded to import std with https://libcxx.llvm.org/Modules.html and
calling `std::printf` after this patch.

I want to put the documentation part to
https://clang.llvm.org/docs/StandardCPlusPlusModules.html in a separate
commit.