Don't download LLVM if its destination exists

Signed-off-by: Dimitar Dobrev <dpldobrev@protonmail.com>

build/llvm/LLVM.lua

@@ -187,6 +187,8 @@ function download_llvm()
   -- check if we already have the file downloaded
   if os.isfile(archive) then
     print("Archive " .. archive .. " already exists.")
+  elseif os.isdir(pkg_name) then
+    print("Directory " .. pkg_name .. " already exists.")
   else
     msg, code = download(base .. archive, archive, true)