Migrate from toolchains_llvm to llvm

MODULE.bazel

@@ -258,18 +258,13 @@ single_version_override(
 )
 
 bazel_dep(
-    name = "toolchains_llvm",
-    version = "1.8.0",
-    dev_dependency = True,
-)
-
-llvm = use_repo_rule("@toolchains_llvm//toolchain:rules.bzl", "llvm")
-
-llvm(
     name = "llvm",
+    version = "0.8.16",
     dev_dependency = True,
-    llvm_versions = {"": "22.1.7"},
 )
+# N.B. We must NOT register llvm's toolchain here. We want to use the
+# auto-detecting toolchain, above ("local_config_cc"). We only use llvm
+# for linting (clang-format) and testing (llvm-objdump).
 
 bazel_dep(
     name = "ruff_prebuilt",

tools/install/libdrake/BUILD.bazel

@@ -494,8 +494,11 @@ drake_py_unittest(
     timeout = "moderate",
     data = [
         ":libdrake.so",
-        "@llvm//:bin/llvm-objdump",
+        "@llvm//tools:llvm-objdump",
     ],
+    env = {
+        "LLVM_OBJDUMP_RLOCATIONPATH": "$(rlocationpath @llvm//tools:llvm-objdump)",
+    },
     deps = [
         "@rules_python//python/runfiles",
         lief_requirement("lief"),

tools/install/libdrake/test/exported_symbols_test.py

@@ -1,4 +1,5 @@
 import bisect
+import os
 import struct
 import subprocess
 import unittest
@@ -249,7 +250,7 @@ def _global_ctors_dtors(symbols):
         manifest = runfiles.Create()
         objdump_all = subprocess.check_output(
             [
-                manifest.Rlocation("llvm/bin/llvm-objdump"),
+                manifest.Rlocation(os.environ["LLVM_OBJDUMP_RLOCATIONPATH"]),
                 LIBDRAKE,
                 "-Mintel",
                 "--no-addresses",

tools/lint/BUILD.bazel

@@ -47,7 +47,7 @@ drake_py_library(
     srcs = ["clang_format.py"],
     data = [
         "//:.clang-format",
-        "@llvm//:bin/clang-format",
+        "@llvm//tools:clang-format",
     ],
     deps = [
         ":module_py",

tools/lint/clang_format.py

@@ -1,18 +1,19 @@
 """Drake's wrapper for the clang-format binary."""
 
 import os
-from pathlib import Path
 import sys
 
 from python import runfiles
 
 
 def get_clang_format_path():
+    # In our runfiles, there should be exactly one path that looks like
+    # "llvm+{module_extension_...}-{version}-{os}-{arch}/bin/clang-format".
+    # Find and return it (or raise an error).
     manifest = runfiles.Create()
-    path = Path(manifest.Rlocation("llvm/bin/clang-format"))
-    if not path.is_file():
-        raise RuntimeError(f"Could not find required clang-format at {path}")
-    return path
+    matches = list(manifest.root().glob("llvm+*/bin/clang-format"))
+    assert len(matches) == 1, repr(matches)
+    return matches[0]
 
 
 def _main():