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

This recommits r341472, which was reverted due to test failures on macos bots.

The issue was that a macos target implies -glldb which, together with
this patch added a -gpubnames switch where there previously wasn't one.
The intentions of those checks was to check that -gpubnames is not
emitted by default so I add an explicit -ggdb arg to those command lines
to get same behavior on all platforms (the fact that -glldb *does* set
-gpubnames is tested by a separate test).

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

llvm-svn: 341564