Only re-generate an object file if the llvm IR (after IRGen) changed.
The check is done based on a MD5 hash of the llvm IR which is stored in a special section in the object file.
This reduces compilation time for multi-threaded whole module compilation if only a small number of files are changed.
The incremental compilation also works for compilations with a single output file. In this case it's all-or-nothing.
