This new DWARFBaseDIE is going to be used for DWARFUnit::GetUnitDIEOnly() as
other DIEs are unavailable that time so the caller should not have methods
available to access them.
This patch is only a mechanical split without any use of it.
Differential revision: https://reviews.llvm.org/D47275
llvm-svn: 333222
Jan Kratochvil