Fix "regression" caused by updating our notion of POD to better match the C++11

Fix "regression" caused by updating our notion of POD to better match the C++11
rules: instead of requiring flexible array members to be POD, require them to
be trivially-destructible. This seems to be the only constraint that actually
matters here (and even then, it's questionable whether this matters).

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198983 91177308-0d34-0410-b5e6-96231b3b80d8