Improve test for merged global debug info by using llvm-dwarfdump.

It's a bit of a tradeoff, since llvm-dwarfdump doesn't print the name of
the global symbol being used as an address in the addressing mode, but
this avoids the dependence on hardcoded set labels that keep changing
(5+ commits over the last few years that each update the set label as it
changes due to other, unrelated differences in output). This could've,
instead, been changed to match the set name then match the name in the
string pool but that would present other issues (needing to skip over
the sets that weren't of interest, etc) and checking that the addresses
(granted, without relocations applied - so it's not the whole story)
match in the two variable location descriptions seems sufficient and
fairly stable here.

There are a few similar other tests with similar label dependence that
I'll update soonish.