Allow repo_mapping parameter to be disabled

With Bzlmod enabled, `repo_mapping` isn't that useful since repositories
are namespaced under their parents. Unfortunately, Bazel errors out if
you try to use it.

This commit conditionally provides the parameter if needed, but allows
it to be disabled.

bindgen/transitive_repositories.bzl

@@ -2,18 +2,38 @@
 
 load("@llvm-raw//utils/bazel:configure.bzl", "llvm_configure", "llvm_disable_optional_support_deps")
 
+DEFAULT_REPO_MAPPING = {
+    "@llvm_zlib": "@zlib",
+}
+LLVM_TARGETS = ["AArch64", "X86"]
+
 # buildifier: disable=unnamed-macro
-def rust_bindgen_transitive_dependencies():
+"""Generate repositories for transitive dependencies needed by the `bindgen` rules.
+
+Args:
+    repo_mapping (dict): Mapping for renaming repositories created by this function.
+        The repository names created by this function are undocumented and subject
+        to change in the future.
+
+"""
+
+def rust_bindgen_transitive_dependencies(repo_mapping = DEFAULT_REPO_MAPPING):
     """Declare transitive dependencies needed for bindgen."""
 
-    llvm_configure(
-        name = "llvm-project",
-        repo_mapping = {"@llvm_zlib": "@zlib"},
-        targets = [
-            "AArch64",
-            "X86",
-        ],
-    )
+    # Bzlmod does not support the `repo_mapping` parameter *at all*, so we have
+    # to only specify it when requested.
+    if repo_mapping == None:
+        llvm_configure(
+            name = "llvm-project",
+            targets = LLVM_TARGETS,
+        )
+    else:
+        llvm_repo_name = repo_mapping.get("llvm-project", default = "llvm-project")
+        llvm_configure(
+            name = llvm_repo_name,
+            repo_mapping = repo_mapping,
+            targets = LLVM_TARGETS,
+        )
 
     # Disables optional dependencies for Support like zlib and terminfo. You may
     # instead want to configure them using the macros in the corresponding bzl