[clang][NFC] Refactor ElaboratedTypeKeyword

This patch moves ElaboratedTypeKeyword before `Type` definition so that the enum is complete where bit-field for it is declared. It also converts it to scoped enum and removes `ETK_` prefix.

GitOrigin-RevId: 4ad2ada5216ee2bb3c334a3233a9ab51f2521b82
2 files changed