[clang] Write original source language when writing and reading AST (PR #209353)

[Nico Weber via cfe-commits]

nico wrote:

> @nico Try this updated test.

I can't easily try this, I just watch my bots :)

But if you run something like `for i in {1..100}; do bin/llvm-lit clang/test/CodeGen/SystemZ/zos-check-lanuage.c; done` and it passes 100 times, it should be good.

Thanks!

https://github.com/llvm/llvm-project/pull/209353