[yaml2obj][ELF] Don't monkeypatch the YAML sections.

[yaml2obj][ELF] Don't monkeypatch the YAML sections.

Previously, we would monkeypatch the vector of YAML::Section's in order
to ensure that the SHT_NULL entry is present. Now we just add it
unconditionally.

The proliferation of small numerical adjustments is beginning to
frighten me, but I can't think of a way having a single point of truth
for them without introducing a whole new layer  of data structures (i.e.
lots of code and complexity) between the YAML and binary ELF formats.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184260 91177308-0d34-0410-b5e6-96231b3b80d8