[lib/ObjectYAML] - Simplify the code that handles Content/Size fields.

This is a follow-up for D89039 patch, which adds a support for
`Content`/`Size` for all sections.

Assuming that all of sections have a support of these 2 fields,
we can simplify and generalize the code.

Depends on D89039

Differential revision: https://reviews.llvm.org/D89120