DebugInfo: Simply & constrain test(s) for PR9600/PR9608

Both these tests were ultimately fixed by the check for
"isIncompleteType" & neither test case was really reduced to a minimal
form. On doing so it becomes apparent that the problem wasn't specific
to templates at all, so I've moved the test case to a more appropriate
test file and added FileCheck verification to it (to show the forward
declaration of the array element type as well as the array alignment and
size being 0 since it cannot be computed). That's about as far down this
rabbithole as I'm willing to go today, so the rest of the un-FileChecked
tests in test/CodeGenCXX/debug-info.cpp will have to go another day
without actually testing anything other than the fact that they don't
crash.

& improve the actually interesting test case in
test/CodeGenCXX/debug-info-templates.cpp which was my original goal (in
preparation for expanding it/fixing some related bugs in non-type
template parameters)