GH-136895: Fixes for pulling LLVM as a release artifact (#141002)

Tools/jit/_llvm.py

@@ -83,6 +83,11 @@ async def _find_tool(tool: str, llvm_version: str, *, echo: bool = False) -> str
     # PCbuild externals:
     externals = os.environ.get("EXTERNALS_DIR", _targets.EXTERNALS)
     path = os.path.join(externals, _EXTERNALS_LLVM_TAG, "bin", tool)
+    # On Windows, executables need .exe extension
+    if os.name == "nt" and not path.endswith(".exe"):
+        path_with_exe = path + ".exe"
+        if os.path.exists(path_with_exe):
+            path = path_with_exe
     if await _check_tool_version(path, llvm_version, echo=echo):
         return path
     # Homebrew-installed executables: