[llvm] [CI] Upstream premerge terraform configuration (PR #117397)

[Mehdi Amini via llvm-commits]

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+# Premerge Infrastructure
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+This folder contains the terraform configuration files that define the GCP
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joker-eph wrote:

`.ci/infrastructure` folder is a bit of a "general" name for this: should we nest it in `.ci/infrastructure/terraform/` instead to leave room for other things here?

Also so far all of that kind of infrastructure has been in Zorg (like [here](https://github.com/llvm/llvm-zorg/tree/main/buildbot/google/terraform) for example), I'm not sure how we think about Zorg vs this right now?

https://github.com/llvm/llvm-project/pull/117397