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

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Ariel-Burton 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!

I played with this a while inserting "sleep 2" between the different stages.  In the original I was able to reproduce your problem.  With the adjustments to set the epoch I was not able to get the test to fail.

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