Support custom LLVM installation path

Add support for explicitly defined LLVM installation location.

This is necessary as MSVC now comes with its own LLVM installation and activating MSVC via vcvars.bat will put LLVM tools on the `PATH` before the local ones.

Author: xhochy

Tools/jit/_llvm.py

@@ -72,6 +72,11 @@ async def _get_brew_llvm_prefix(llvm_version: str, *, echo: bool = False) -> str
 
 @_async_cache
 async def _find_tool(tool: str, llvm_version: str, *, echo: bool = False) -> str | None:
+    # Explicitly defined LLVM installation location
+    if (llvm_root := os.getenv("LLVM_ROOT")) is not None:
+        path = os.path.join(llvm_root, "bin", tool)
+        if await _check_tool_version(path, llvm_version, echo=echo):
+            return path
     # Unversioned executables:
     path = tool
     if await _check_tool_version(path, llvm_version, echo=echo):