[yaml2obj/obj2yaml] - Make RawContentSection::Content and RawContentSection::Size optional

This is a follow-up for D62809.

Content and Size fields should be optional as was discussed in comments
of the D62809's thread. With that, we can describe a specific string table and
symbol table sections in a more correct way and also show appropriate errors.

The patch adds lots of test cases where the behavior is described in details.

Differential revision: https://reviews.llvm.org/D62957

llvm-svn: 362931