[C++] Add static reflection / stringification for enums (#6499)

[ajrh1]

Add an EnumTraits<T> class, templated on the enum type, largely as proposed in #6499, except in the generated namespace.

Add overloaded EnumValTraits(T) to return the traits class via ADL.

Add GetEnumName(T t) helper for the common case of finding the enum string name from its value.

Changed the cpp17 test and expected test output for stringified enums.

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[ajrh1]

Hi @dbaileychess @aardappel ... I wonder this seems to be stuck. Woukd you know is there anything missing to get this reviewed?

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