[yaml2obj] - Teach tool to produce SHT_GROUP section with a custom type.

Currently, it is possible to use yaml2obj for producing SHT_GROUP sections
of type GRP_COMDAT. For LLD test case I need to produce an object with
a broken (different from GRP_COMDAT) type.

The patch teaches tool to do such things.

Differential revision: https://reviews.llvm.org/D50761

llvm-svn: 339764