[Bazel] Add example for bzlmod usage with custom commits

[aaronmondal]

Summary

Adds a copy-pasteable example for the bzlmod build when using custom commits.

[aaronmondal]

Continuation of https://reviews.llvm.org/D137008.

utils/bazel/examples/bzlmod_custom_commit/.bazelrc

+common --repo_env=BAZEL_CXXOPTS='-std=c++17:-O3'
+common --experimental_enable_bzlmod
+
+# TODO(aaronmondal): Only for testing during review. Remove before merging this patch.
+common --registry=https://raw.githubusercontent.com/eomii/bazel-eomii-registry/llvm-project-overlay

utils/bazel/examples/bzlmod_custom_commit/.bazelversion

+5.3.2

utils/bazel/examples/bzlmod_custom_commit/MODULE.bazel

+bazel_dep(name = "llvm-project-overlay", version = "16.0.0-bcr.0")
+
+llvm_project_overlay = use_extension(
+    "@llvm-project-overlay//utils/bazel:extensions.bzl",
+    "llvm_project_overlay",
+)
+
+# TODO(aaronmondal): After the other patches in the bzlmod patch series are
+# committed, update <custom_commit> and <some_hash> and submit this example.
+llvm_project_overlay.configure(
+    targets = ["X86"],  # Using a subset of Clang targets to reduce build times.
+    commit = "<custom_commit>",
+    sha256 = "<some_hash>",
+    patches = ["//:mallinfo2_patch.diff"],
+)
+
+use_repo(
+    llvm_project_overlay,
+    "llvm-project",
+)

utils/bazel/examples/bzlmod_custom_commit/mallinfo2_patch.diff

+diff --git a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h b/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
+index 45945510d..88c0afb66 100644
+--- a/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
++++ b/utils/bazel/llvm-project-overlay/llvm/include/llvm/Config/config.h
+@@ -159,6 +159,9 @@
+ /* Define to 1 if you have the `mallinfo' function. */
+ /* HAVE_MALLINFO defined in Bazel */
+
++/* Define to 1 if you have the `mallinfo2` function. */
++#define HAVE_MALLINFO2 1
++
+ /* Define to 1 if you have the <malloc/malloc.h> header file. */
+ /* HAVE_MALLOC_MALLOC_H defined in Bazel */
+