Mark static const class/struct members as constexpr.

This change fixes declarations that have initial values but are
technically not definitions by marking them constexpr (which counts as a
definition). This enables, among other things, the modified constants to
be passed into functions and function templates that accept arguments by
reference. Without this change, such functions would cause linker
errors.

Change-Id: Id8798c69270f053ae29b02bef4c9fee616e2b17c
PiperOrigin-RevId: 307129442
1 file changed