Sink DwarfUnit::LabelBegin down into DwarfCompileUnit since that's the only place it's needed.
Drop DwarfCompileUnit::getLocalLabel* in favor of just mapping through the skeleton explicitly.

Confusing to do this two different ways - I'm not too wedded to either
one, but here goes.
Docs: update va_arg example with valid x86_64 va_list type.

The given example was overflowing its alloca and segfaulting if actually run on
x86, so it's a good idea to provide something that works there too.

Patch by Ramkumar Ramachandra.