[llvm] [bazel][NFC] Avoid set for older bazel versions (PR #170758)

[Jordan Rupprecht via llvm-commits]

https://github.com/rupprecht created https://github.com/llvm/llvm-project/pull/170758

The `set` type is only available on Bazel 8.1, which is newer than some projects. This gives some time for downstream users of LLVM to migrate to Bazel 8.1 or newer.

e.g. https://github.com/openxla/xla/actions/runs/19943322161/job/57186478292

>From bc37c61516435d2c6c1be80b369fab53f7ed3ee8 Mon Sep 17 00:00:00 2001
From: Jordan Rupprecht <rupprecht at google.com>
Date: Thu, 4 Dec 2025 14:01:38 -0800
Subject: [PATCH] [bazel][NFC] Avoid set for older bazel versions

---
 utils/bazel/configure.bzl | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/utils/bazel/configure.bzl b/utils/bazel/configure.bzl
index adbd3c6539037..09861b2f00911 100644
--- a/utils/bazel/configure.bzl
+++ b/utils/bazel/configure.bzl
@@ -27,7 +27,6 @@ DEFAULT_TARGETS = [
     "XCore",
 ]
 
-
 MAX_TRAVERSAL_STEPS = 1000000  # "big number" upper bound on total visited dirs
 
 def _overlay_directories(repository_ctx):
@@ -44,7 +43,9 @@ def _overlay_directories(repository_ctx):
     for _ in range(MAX_TRAVERSAL_STEPS):
         rel_dir = stack.pop()
 
-        overlay_dirs = set()
+        # TODO: `set()` is only available in bazel 8.1.
+        # Use `set()` after downstream users are on more recent versions.
+        overlay_dirs = {}
 
         # Symlink overlay files, overlay dirs will be handled in future iterations.
         for entry in overlay_root.get_child(rel_dir).readdir():
@@ -53,7 +54,7 @@ def _overlay_directories(repository_ctx):
 
             if entry.is_dir:
                 stack.append(full_rel_path)
-                overlay_dirs.add(name)
+                overlay_dirs[name] = None
             else:
                 src_path = overlay_root.get_child(full_rel_path)
                 dst_path = target_root.get_child(full_rel_path)
@@ -62,7 +63,7 @@ def _overlay_directories(repository_ctx):
         # Symlink source dirs (if not themselves overlaid) and files.
         for src_entry in src_root.get_child(rel_dir).readdir():
             name = src_entry.basename
-            if name in overlay_dirs:
+            if name in overlay_dirs.keys():
                 # Skip: overlay has a directory with this name
                 continue