[llvm] [Github] Add initial version of precommit checks (PR #80951)

[Tom Stellard via llvm-commits]

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@@ -0,0 +1,47 @@
+name: "Precommit tests"
+
+permissions:
+  contents: read
+
+on:
+  pull_request:
+    branches:
+      - main
+    paths:
+      - '.github/workflows/precommit.yaml'
+      - 'llvm/**'
+
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tstellar wrote:

Do we need to define concurrency groups?  What's the default behavior if a PR gets updated while its previous version is still being tested?

https://github.com/llvm/llvm-project/pull/80951