Try out FileCheck's new (in r212810) -implicit-check-not in a DebugInfo test.

Just tried this on a few tests and this was the only one that was
easily ported to use the new feature, so we'll go with that for now.
Hopefully can act as inspiration/reminder for other tests.

Not all debug info tests need to check for every DW_TAG or NULL child
terminator, but perhaps they should (just to ensure they don't accidentally
end up with tags nested inside other tags without the test failing, for example)

llvm-svn: 213092