[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:

It isn’t obvious to me why a terraform script that somewhere calls a premerge script in the mono repo is more coupled than any infra setup in Zerg that calls a similar script in the mono repo (or even call cmake+ninja, which isn’t different than a script entry point really).

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