Modify EnumBase to better support the namespacing of toolchain (#3156)

The different approach to Names avoids the issues with trying to define
a static member (or also member function) of the templated instance of
Carbon::Internal::EnumBase from a non-enclosing namespace such as
Carbon::SemIR.

Note, I'm trying to do this from the cpp file. An alternative might be
to do `inline constexpr llvm::StringLiteral Names[]` in the .h file, but
I think concerns had been raised about that needing deduplication.

explorer/ast/ast_rtti.cpp

@@ -15,6 +15,5 @@ CARBON_DEFINE_ENUM_CLASS_NAMES(AstRttiNodeKind) = {
   CARBON_ENUM_NAME_FUNCTION(C##Kind) {                                 \
     return AstRttiNodeKind(static_cast<const C##Kind&>(*this)).name(); \
   }
-CARBON_AST_FOR_EACH_ABSTRACT_CLASS(DEFINE_NAME_FUNCTION)
 
 }  // namespace Carbon