IssueTracker

Support for concurrent code generation when using the LLVM backend has been implemented as part of b/153803432, but Subzero and our use of it still requires a global lock.

With that fixed we would be able to have the Nucleus class hold a thread-local pointer to a private implementation of the JIT "engine" (i.e. have the equivalent of LLVMReactor's JITBuilder), and then share more logic for things like function finalization. It would also be a step towards tiered compilation, where we would decide at run-time which JIT engine to use.