[DebugInfo] Fix emitting DWARF64 .debug_names sections (16/19).

The patch fixes emitting the unit length field in the header of
the table and offsets to the entry pool. Note that while the patch
changes the common method to emit offsets, in fact, nothing is changed
for Apple accelerator tables, because we do not yet support DWARF64 for
those targets.

Differential Revision: https://reviews.llvm.org/D87023