[ORC] Switch RTDyldObjectLinkingLayer to take a unique_ptr<MemoryBuffer> rather

[ORC] Switch RTDyldObjectLinkingLayer to take a unique_ptr<MemoryBuffer> rather
than a shared ObjectFile/MemoryBuffer pair.

There's no need to pre-parse the buffer into an ObjectFile before passing it
down to the linking layer, and moving the parsing into the linking layer allows
us remove the parsing code at each call site.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@325725 91177308-0d34-0410-b5e6-96231b3b80d8