[clang] [TEST-ONLY] Make sure clang-extdef-mapping don't infer database (PR #195196)

[Balázs Benics via cfe-commits]

https://github.com/steakhal approved this pull request.

I think we collectively spend weeks debugging these mysterious compilation commands.
I was bitten by this with clang-tidy in rdar://165014675
Now, It's you. I really wish there was something written to stderr when this auto-detection happens.

The remaining problem I can see here is that we wouldn't know in upcoming tests that these double-dashes have some special meaning. While these fixes what we have today, it wouldn't help with future tests. From what I can see, this will remain a recurring problem.

https://github.com/llvm/llvm-project/pull/195196