[Build] Symlink compile_commands.json to root dir (#7305)

Symlink the `compile_commands.json` file from the build directory to the
root repository directory.

So we no longer need to elaborate in the README on where to find it. 

Everything just works out of the box for VSCode IntelliSense and Clangd.

Author: ShawnZhong

README.md

@@ -119,15 +119,17 @@ Alternatively, follow these steps to build LLVM from source manually.
   Without this, every invocation of `pip install` uses a different symlink to
   cmake, and this forces ninja to rebuild most of the `.a` files.
 
-- vscode intellisense has some difficulty figuring out how to build Triton's C++
-  (probably because, in our build, users don't invoke cmake directly, but
-  instead use setup.py).  Teach vscode how to compile Triton as follows.
+- The build system creates a `compile_commands.json` file under the Triton repo
+  directory. This file is used by VSCode IntelliSense and clangd to provide
+  code completion and other features for C++ code.
+
+  If IntelliSense does not work, you can try the following steps:
 
     - Do a local build. Run command `pip install -e .`
     - Get the full path to the `compile_commands.json` file produced by the build:
       `find ./build -name 'compile_commands.json' | xargs readlink -f`.
       You might get a full path similar to `/Users/{username}/triton/build/cmake.macosx-11.1-arm64-cpython-3.12/compile_commands.json`
-    - In vscode, install the
+    - In VSCode, install the
       [C/C++
       extension](https://marketplace.visualstudio.com/items?itemName=ms-vscode.cpptools),
       then open the command palette (`Shift + Command + P` on Mac, or `Shift +

setup.py

@@ -525,6 +525,8 @@ def build_extension(self, ext):
         subprocess.check_call(["cmake", "--build", "."] + build_args, cwd=cmake_dir)
         subprocess.check_call(["cmake", "--build", ".", "--target", "mlir-doc"], cwd=cmake_dir)
 
+        update_symlink(Path(self.base_dir) / "compile_commands.json", cmake_dir / "compile_commands.json")
+
 
 def download_and_copy_dependencies():
     nvidia_version_path = os.path.join(get_base_dir(), "cmake", "nvidia-toolchain-version.json")