[llvm] [bazel] Add potential bzlmod inclusion example (PR #173124)

[Matt Clarkson via llvm-commits]

================
@@ -0,0 +1,29 @@
+# Replace with the LLVM commit you want to use.
+LLVM_COMMIT = "2d1bc9f5a212dc4c8fa14a5741af2a7a5e60ff36"
+
+# The easiest way to calculate this for a new commit is to set it to empty and
+# then run a bazel build and it will report the digest necessary to cache the
+# archive and make the build reproducible.
+LLVM_INTEGRITY = "sha256-KST3jta9pEHT0igBUxNaUoJysbefc0yBJ22UEzWcINs="
+
+bazel_dep(name = "llvm-raw")
+archive_override(
+    module_name = "llvm-raw",
+    integrity = LLVM_INTEGRITY,
+    patch_cmds = [
+        "cat utils/bazel/MODULE.bazel | grep -v '\"llvm-raw\"' | sed 's|llvm-project-overlay|llvm-raw|g' > MODULE.bazel",
+    ],
----------------
mattyclarkson wrote:

This will only work for the root repository as overrides do not work transitively. 

My suggestion is that we create the extension _outside_ of this project, maybe at `bazel-contrib/llvm-project` that does:

```py
# Bikeshedding welcome on names, API, etc
bazel_dep(name = "llvm-project", version = "1.0.0")
llvm = use_extension("@llvm-project//:extension.bzl", llvm)
llvm.download(
    version = "21.0.1",  # Could support semantic version modifiers?
    targets = ["..."],
    # template = "https://github.com/llvm/llvm-project/archive/{version}.tar.gz",
    # integrity = "...",
)
use_repo(llvm, llvm = "llvm-21.0.1")  # `bazel mod tidy` to generate
# Use `@llvm//clang`, etc
```

This would have the following benefits:
  - Bazel would initially only download a thin Bazel module that contains the extension
  - The extension can collapse LLVM version downloads across the graph
  - It could combine repositories that have the same version and target subsets
  - Modules can used different versions of LLVM across the graph

Underneath, the extension would modify, hermetically, the downloaded repository to move the `MODULE.bazel` and configure the correct targets.

My feeling is that most projects will want to have a particular LLVM version rather than collapsing to a single LLVM version across the `bzlmod` graph?

https://github.com/llvm/llvm-project/pull/173124