This change introduces the symbol
ABSL_INTERNAL_NEED_REDUNDANT_CONSTEXPR_DECL
to guard redundant declarations of static constexpr data
members that are needed prior to C++17.

This change also introduces the symbol
ABSL_INTERNAL_CPLUSPLUS_LANG, which is supposed to be set
to the same value as __cplusplus, except it uses _MSVC_LANG
on MSVC so that the value is correct on MSVC.

Neither of these new symbols should be used outside of Abseil.

Fixes #1191

PiperOrigin-RevId: 453923908
Change-Id: I1316c52c19fa0c168b93cced0c817e4cb7c9c862