Give ExecutionEngine of top level buffers.

Long term the idea if for the engine to not own the buffers, but for now
this is consistent with the rest of the API.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216484 91177308-0d34-0410-b5e6-96231b3b80d8
Pass a MemoryBufferRef when we can avoid taking ownership.

The attached patch simplifies a few interfaces that don't need to take
ownership of a buffer.

For example, both parseAssembly and parseBitcodeFile will parse the
entire buffer before returning. There is no need to take ownership.

Using a MemoryBufferRef makes it obvious in the type signature that
there is no ownership transfer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@216488 91177308-0d34-0410-b5e6-96231b3b80d8