[yaml2obj][ELF] Ensure more fields are zero'd.

[yaml2obj][ELF] Ensure more fields are zero'd.

I was spotting garbage in the output. I'd like to just zero the entire
ELFYAML::Section to be sure, but it contains non-POD types. (I'm also
trying to avoid bloating the ELFYAML::Foo classes with a bunch of
constructor code).

No test, since this is by its very nature unpredictable. I'm pretty sure
that one of the sanitizers would catch it immediately though.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184160 91177308-0d34-0410-b5e6-96231b3b80d8