[llvm] [Github][CI] Add separate container for code-format premerge job (PR #161083)

[Aiden Grossman via llvm-commits]

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@@ -0,0 +1,72 @@
+FROM docker.io/library/ubuntu:24.04 AS llvm-downloader
+
+ENV LLVM_VERSION=21.1.1
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boomanaiden154 wrote:

Now that we have explicit control, this should be `21.1.0` to be consistent with what we use for `clang-format`.

https://github.com/llvm/llvm-project/pull/161083