Enable DWARF accelerator tables by default when tuning for lldb (-glldb => -gpubnames)

Summary:
DWARF v5 accelerator tables provide a considerable performance
improvement for lldb and will make the default -glldb behavior same on
all targets (right now we emit apple tables on apple targets, but these
are not controlled by -gpubnames, only by -glldb).

Reviewers: dblaikie

Subscribers: probinson, clayborg, JDevlieghere, aprantl, cfe-commits

Differential Revision: https://reviews.llvm.org/D51576

llvm-svn: 341472