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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@336385 91177308-0d34-0410-b5e6-96231b3b80d8