Practically all current compilers fully support C++17 by now, with the exception of `filesystem` (which we won't use).
PiperOrigin-RevId: 281494879
Change-Id: Ia0ef650ede4e5907e4a6421b8ad11743c8bae88f
These where inadvertently introduced in an internal cleanup change.
This change also removes a C++17-ism in var_proto.h. To make things easier for
downstream projects, we should stick to C++11 for the time being.
PiperOrigin-RevId: 271117700
Change-Id: I4eaacec88be16e1a561d3f77a61acce0a1af0b9d
