DWARFDIE split out to DWARFBaseDIE

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