Clang-15 deprecated warnings

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

.../3rd_party/JUCE/modules/juce_core/containers/juce_Variant.cpp:496:43: 
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.