Trans should use LLVM in a threadsafe fashion #6511
Description
I recently wrote some unit tests for rusti, and because unit tests are by default run in parallel, it exercised the trans stage of the compiler in parallel. Things quickly fell apart and this never worked.
It would be nice to be able to invoke the compiler in parallel from within one process. This may just entail throwing a huge lock around trans for now, but it would also be awesome to use LLVM in a threadsafe way