[yaml2obj] Initial ELF section support.

The current functionality is extremely basic and a bit rough around the
edges, but it will flesh out in future commits.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183953 91177308-0d34-0410-b5e6-96231b3b80d8
[yaml2obj] Add sh_addr via `Address` key.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183954 91177308-0d34-0410-b5e6-96231b3b80d8
[yaml2obj] Add support for specifying raw section content.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@183955 91177308-0d34-0410-b5e6-96231b3b80d8