* Use constexpr more with VC++ 2017

Chrome's official builds have over 170 dynamic initializers for
variables of the form *::TableStruct::aux. Defining
PROTOBUF_CONSTEXPR_VAR to be constexpr for VS 2017 gets rid of all of
these and saves about 10 KB of binary size.

* Update generated_message_table_driven.h

Restore accidentally deleted line.