Correct behavior of VLA extension diagnostic in C89 mode

Post-commit feedback (https://reviews.llvm.org/D156565#4654773) found
that the changes in 84a3aadf caused us
to diagnose use of VLAs in C89 mode by default which was an unintended
change.

This adds -Wvla-cxx-extension as a warning group and adds the C++-
specific warnings to it while leaving the C warnings under
-Wvla-extension. -Wvla-cxx-extension is then added to -Wall.