llvm: fix submodule path

- Use official llvm repo
- Move llvm to 3rdparty
- Ignore SPIRV submodules in CI

.ci/build-linux.sh

@@ -12,7 +12,7 @@ fi
 # Pull all the submodules except llvm
 # Note: Tried to use git submodule status, but it takes over 20 seconds
 # shellcheck disable=SC2046
-git submodule -q update --init $(awk '/path/ && !/llvm/ { print $3 }' .gitmodules)
+git submodule -q update --init $(awk '/path/ && !/llvm/ && !/SPIRV/ { print $3 }' .gitmodules)
 
 mkdir build && cd build || exit 1