[yaml2obj] - Teach yaml2obj to produce SHT_GROUP section with a custom Info field.

This allows to set custom Info field value for SHT_GROUP sections.

It is useful to allow this because we would be able to replace at least one binary
object committed in LLD and replace it with the yaml2obj based test.

Differential revision: https://reviews.llvm.org/D50776

llvm-svn: 339772