Switch llvm-raw to a git_override rule (#6854)

By using git_override, we get some validation from the sha, while
removing the sha256 on the .tar.gz which has been brittle lately. Note
the difference between downloading via sha is this still locally
validates content.

Versus something like #6844, this doesn't update the llvm version, just
how we get it.

Assisted-by: Google Antigravity with Gemini

MODULE.bazel

@@ -26,11 +26,6 @@ versions.
 
 module(name = "carbon")
 
-http_archive = use_repo_rule(
-    "@bazel_tools//tools/build_defs/repo:http.bzl",
-    "http_archive",
-)
-
 bazel_dep(name = "abseil-cpp", version = "20260107.1")
 bazel_dep(name = "bazel_skylib", version = "1.9.0")
 bazel_dep(name = "boost.unordered", version = "1.90.0.bcr.1")
@@ -78,26 +73,25 @@ bazel_dep(name = "zstd", version = "1.5.7.bcr.1", repo_name = "llvm_zstd")
 # llvm-project
 ###############################################################################
 
-# We pin to specific upstream commits and try to track top-of-tree reasonably
-# closely rather than pinning to a specific release.
-# HEAD as of 2026-02-12.
-llvm_project_version = "b20d7d0278059735bb8eb538ac51e13b58895c1a"
-
 # Load a repository for the raw llvm-project, pre-overlay.
-http_archive(
-    name = "llvm-raw",
+bazel_dep(name = "llvm-raw")
+git_override(
+    module_name = "llvm-raw",
     build_file_content = "# empty",
-    integrity = "sha256-sRZjS3KippWoaVcr1mWtJ1kFZ+jrjb/KoNkzh+N8HOM=",
-    patch_args = ["-p1"],
+    # We pin to specific upstream commits and try to track top-of-tree
+    # reasonably closely rather than pinning to a specific release.
+    # HEAD as of 2026-02-12.
+    commit = "b20d7d0278059735bb8eb538ac51e13b58895c1a",
+    patch_cmds = ["echo \"module(name='llvm-raw')\" > MODULE.bazel"],
+    patch_strip = 1,
     patches = [
-        "@carbon//bazel/llvm_project:0001_Patch_for_mallinfo2_when_using_Bazel_build_system.patch",
-        "@carbon//bazel/llvm_project:0002_Added_Bazel_build_for_compiler_rt_fuzzer.patch",
-        "@carbon//bazel/llvm_project:0004_Introduce_basic_sources_exporting_for_libunwind.patch",
-        "@carbon//bazel/llvm_project:0005_Introduce_basic_sources_exporting_for_libcxx_and_libcxxabi.patch",
-        "@carbon//bazel/llvm_project:0009_Introduce_starlark_exporting_compiler-rt_build_information.patch",
+        "//bazel/llvm_project:0001_Patch_for_mallinfo2_when_using_Bazel_build_system.patch",
+        "//bazel/llvm_project:0002_Added_Bazel_build_for_compiler_rt_fuzzer.patch",
+        "//bazel/llvm_project:0004_Introduce_basic_sources_exporting_for_libunwind.patch",
+        "//bazel/llvm_project:0005_Introduce_basic_sources_exporting_for_libcxx_and_libcxxabi.patch",
+        "//bazel/llvm_project:0009_Introduce_starlark_exporting_compiler-rt_build_information.patch",
     ],
-    strip_prefix = "llvm-project-{0}".format(llvm_project_version),
-    urls = ["https://github.com/llvm/llvm-project/archive/{0}.tar.gz".format(llvm_project_version)],
+    remote = "https://github.com/llvm/llvm-project.git",
 )
 
 # Apply the overlay to produce llvm-project.

MODULE.bazel.lock

@@ -324,7 +324,7 @@
         "envVariables": {},
         "generatedRepoSpecs": {
           "llvm-project": {
-            "repoRuleId": "@@+_repo_rules+llvm-raw//utils/bazel:configure.bzl%llvm_configure",
+            "repoRuleId": "@@llvm-raw+//utils/bazel:configure.bzl%llvm_configure",
             "attributes": {
               "targets": [
                 "AArch64",
@@ -337,7 +337,7 @@
           [
             "",
             "llvm-raw",
-            "+_repo_rules+llvm-raw"
+            "llvm-raw+"
           ]
         ]
       }