Quite embarassingly, the test that the WTF-8 decoder rejects surrogate
pairs was broken: the trailing surrogate was invalid.  (The range of the
second byte for leading surrogates is [A0,AF], and for trailing is
[B0,BF]).  Of course the actual functionality was broken, because the
code that detected surrogate pairs called IsSurrogatePair with swapped
arguments.

Bug: v8:12868
Change-Id: Icab5e2e4e200afb3d34f478ab4f98b739ada5645
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3723497Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Andy Wingo <wingo@igalia.com>
Cr-Commit-Position: refs/heads/main@{#81376}