[llvm] [CMake] Add root CMakeLists.txt to allow direct CMake execution from root (PR #152753)

[Shaoce SUN via llvm-commits]

https://github.com/sunshaoce created https://github.com/llvm/llvm-project/pull/152753

Add a top-level `CMakeLists.txt` file to make llvm-project consistent with many other projects that place their `CMakeLists.txt` in the repository root. This change allows CMake to be invoked directly from the root directory, preventing confusion for users who cannot find a root CMake file and improving the contributor experience.

>From a8fe9f385f88ed25c9ddb52ecae8225c9687c073 Mon Sep 17 00:00:00 2001
From: Shao-Ce SUN <sunshaoce at outlook.com>
Date: Sat, 9 Aug 2025 00:11:03 +0800
Subject: [PATCH] [CMake] Add root CMakeLists.txt to allow direct CMake
 execution from root

---
 CMakeLists.txt | 4 ++++
 1 file changed, 4 insertions(+)
 create mode 100644 CMakeLists.txt

diff --git a/CMakeLists.txt b/CMakeLists.txt
new file mode 100644
index 0000000000000..b908b8a7082bd
--- /dev/null
+++ b/CMakeLists.txt
@@ -0,0 +1,4 @@
+cmake_minimum_required(VERSION 3.20.0)
+
+# Primary CMake config is the CMakeLists.txt file in the llvm directory
+add_subdirectory(llvm)