The WasmStringConcat builtin delegates all the work to yet another
builtin (StringAdd_CheckNone); so from optimized code we might as
well call the latter directly.

Bug: v8:12868
Change-Id: I3876ce1d6341befac5d49ba02c4af47637ad4ba3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3823124Reviewed-by: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82436}