Revert "[ORC] Switch from shared_ptr to unique_ptr for addModule methods."

This reverts commit r327566, it breaks
test/ExecutionEngine/OrcMCJIT/test-global-ctors.ll.

The test doesn't crash with a stack trace, unfortunately. It merely
returns 1 as the exit code.

ASan didn't produce a report, and I reproduced this on my Linux machine
and Windows box.

llvm-svn: 327576