Clang-15 deprecated warnings

Compiling JUCE (develop) with clang-15 on Windows in C++20 mode results in the following warnings:

warning: out-of-line definition of constexpr static data member is redundant in C++17 
and is deprecated [-Wdeprecated]

constexpr var::VariantType var::Instance::attributesVoid;

// Same warning for all attribute definitions (11)
// ...

Since the develop branch is on C++17 now, this should be an easy fix.