[yaml2obj][ELF] Just let this class own its buffer.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184456 91177308-0d34-0410-b5e6-96231b3b80d8
[yaml2obj][ELF] Start factoring into "single point of truth".

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184457 91177308-0d34-0410-b5e6-96231b3b80d8