Description
Good or bad, Clang supports VLAs in C++ code. However, this comes as quite the surprise to users (https://ddanilov.me/default-non-standard-features/ among plenty of others) and there are more idiomatic alternatives that users should consider using in C++. I think we should enable -Wvla-extension by default in C++ so that users are made aware of this extension, especially given the security implications around stack overflows with VLAs that can be mitigated by heap allocations through C++ containers.
One challenge to this is that our existing warnings around VLAs and extensions are awkward and should be addressed: https://godbolt.org/z/eEnMsr4zG (we claim we're making a VLA as an extension and then saying nevermind, we decided to make it a constant array, also as an extension).