[ORC] In CompileOnDemandLayer2, clone modules on to different contexts by

writing them to a buffer and re-loading them.

Also introduces a multithreaded variant of SimpleCompiler
(MultiThreadedSimpleCompiler) for compiling IR concurrently on multiple
threads.

These changes are required to JIT IR on multiple threads correctly.

No test case yet. I will be looking at how to modify LLI / LLJIT to test
multithreaded JIT support soon.

llvm-svn: 336385