[DebugInfo] Clean up DIEUnit. NFC.

This removes members of the DIEUnit class which were used only in unit
tests. Note also that child classes shadowed some of these methods,
namely, getDwarfVersion() was overridden in DwartfUnit and getLength()
was overridden in DwarfCompileUnit.

Differential Revision: https://reviews.llvm.org/D85436