Clang doesn't emit debug info for labels (Clang PR14500). XFAIL a
bunch of GDB tests that rely on debug info for labels.

For some reason gdb.linespec/ls-expr.exp gathered all tests into a
dictionary and then ran them. This made it hard to XFAIL just the
right tests. I refactored this to execute the tests directly, removing
the dictionary so I could XFAIL the right tests. Is there a reason it
would've been written that way? Does my patch break it in some way?