[lldb/DWARF/test] Freshen up debug_names tests

These tests used "clang -mllvm -accel-tables=Dwarf" as a way to
guarantee that clang will emit the debug_names table. Unfortunately,
a change it clang made that insufficient (-gpubnames is required now
too), which rendered these tests ineffective. Since lldb automatically
falls back to the manual index, the tests didn't fail and this change
went largely unnoticed.

This patch updates the tests to really use debug_names (-gdwarf-5
-gpubnames) is the combination that works now, and it adds additional
checks to ensure the section is actually emitted.

Fortunately, no regressions crept in while these tests were disabled.