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

[Nathan Gauër 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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Keenuts wrote:

I agree with the points made so far. Is is true that most of what's proposed here is mostly internals for infra management so this could indeed live in llvm-zorg.

And since a googler has to run a `terraform apply` to any change anyway, having them in this repo or the other doesn't bring IMO any sync/atomicity benefit.

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