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
5 files changed