Add LLVM as a git submodule

[xermicus]

I'm not entirely sure whether the lock file is realy needed. We include the llvm commit slug in the resolc version string which should be enough information for reproducible build.

• Remove the LLVM.lock mechanism and related expensive cloneing and checkout tests in the LLVM builder
• Add it as a submodule
• Change the CI actions and workflows to make sure the submodule always gets initialized