[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:

Zorg (from when I've hacked on it) mostly contains Python scripts that define testing workflows for the various builders. The equivalent testing workflows for premerge are defined in the monorepo (currently shell scripts in `.ci`, eventually GHA workflow definitions in `.github`), which leads me to the conclusion that premerge is more tightly coupled to the monorepo.

Putting premerge infra in Zorg seems a bit out of place to me. Currently, it solely contains postcommit configurations/infra. We already have a spot where all of the in-tree premerge scripts and what not live, and that's `.ci/` in the monorepo.

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