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

[Aiden Grossman 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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boomanaiden154 wrote:

The main difference is that all of the associated scripts/build configurations/setup is defined in the monorepo. The premerge CI scripts are already here, and all the GHA workflows will have to be in the monorepo. So yes, I think the premerge infra is significantly more coupled to the monorepo than the postcommit infra in zorg.

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