Group forward declarations by namespace in GeneratedSerializers.h to simplify the generated header and improve readability.
Pull request: https://github.com/WebKit/WebKit/pull/20677
Committed 270908@main (54f2b1ca9dfd): <https://commits.webkit.org/270908@main> Reviewed commits have been landed. Closing PR #20677 and removing active labels.
<rdar://problem/118574783>