Use ASCIILiteral instead of `const char*` for Supplementable key.
Pull request: https://github.com/WebKit/WebKit/pull/22680
Committed 272955@main (67e8a2009c53): <https://commits.webkit.org/272955@main> Reviewed commits have been landed. Closing PR #22680 and removing active labels.
<rdar://problem/120881891>