[DebugInfo] Make DIELocList::SizeOf() more explicit. NFCI.

DIELocList is used with a limited number of DWARF forms, see the only
place where it is instantiated, DwarfCompileUnit::addLocationList().

The patch marks the unexpected execution path in DIELocList::SizeOf()
as unreachable, to reduce ambiguity.

Differential Revision: https://reviews.llvm.org/D84092