Pass a std::unique_ptr<MemoryBuffer>& to getLazyBitcodeModule.

By taking a reference we can do the ownership transfer in one place instead of
expecting every caller to do it.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216492 91177308-0d34-0410-b5e6-96231b3b80d8