[clang][NFC] Improve locality of recently refactored enums (#70943)

This patch moves definition of recently refactored enums closer to the
types where they were originally defined. Since they are scoped enums at
namespace scope now, they can be forward-declared.

Refactorings in question are:
aaba3761
50dec541
b120fe8d
ae7b20b5
4ad2ada5
49fd28d9