add llvm18 as a separate module instead of sdk extension

this is because freedesktop 25.08 doesn't host the llvm18 sdk extension anymore, so we have to add it as a module instead

flatpak/io.github.hedge_dev.unleashedrecomp.json

@@ -3,7 +3,6 @@
   "runtime": "org.freedesktop.Platform",
   "runtime-version": "25.08",
   "sdk": "org.freedesktop.Sdk",
-  "sdk-extensions" : [ "org.freedesktop.Sdk.Extension.llvm20" ],
   "finish-args": [
     "--share=network",
     "--socket=wayland",
@@ -16,6 +15,22 @@
     "--filesystem=/mnt"
   ],
   "modules": [
+    {
+      "name": "llvm",
+      "buildsystem": "simple",
+      "build-commands": [
+        "mkdir -p /usr/lib/sdk/llvm18",
+        "tar clang+llvm-18.1.8-x86_64-linux-gnu-ubuntu-18.04.tar --strip-components=1 -C /usr/lib/sdk/llvm18/"
+      ],
+      "sources": [
+        {
+          "type": "archive",
+          "url": "https://github.com/llvm/llvm-project/releases/download/llvmorg-18.1.8/clang+llvm-18.1.8-x86_64-linux-gnu-ubuntu-18.04.tar.xz",
+          "sha256": "54ec30358afcc9fb8aa74307db3046f5187f9fb89fb37064cdde906e062ebf36"
+        }
+      ],
+      "cleanup": [ "*" ] 
+    },
     {
       "name": "UnleashedRecomp",
       "buildsystem": "simple",
@@ -35,8 +50,8 @@
         }
       ],
       "build-options": {
-        "append-path": "/usr/lib/sdk/llvm20/bin",
+        "append-path": "/usr/lib/sdk/llvm18/bin",
-        "prepend-ld-library-path": "/usr/lib/sdk/llvm20/lib",
+        "prepend-ld-library-path": "/usr/lib/sdk/llvm18/lib",
         "build-args": [
           "--share=network"
         ]