Code cleanup: Remove DWARFDebugInfoEntry::m_empty_children

It merges DWARFDebugInfoEntry's m_empty_children into m_has_children.
m_empty_children was implemented by rL144983.

As Greg confirmed m_has_children was used to represent what was in the DWARF in
the byte that follows the DW_TAG. m_empty_children was used for DIEs that said
they had children but actually only contain a single NULL tag. It is fine to
not differentiate between the two.

Also changed assert()->lldbassert() for m_abbr_idx 16-bit overflow check as
that could be a tough bug to catch if it ever happens.

I have checked all calls of HasChildren() that this change should not matter to
them. The code even wants to know if there are any children - it does not
matter how the children presence is coded in the binary.

Patch written based on suggestions by Greg Clayton.

Differential Revision: https://reviews.llvm.org/D53321

llvm-svn: 344644