The last use of V8's SplayTree was in diagnostics and is now replaced
by std::map.

Bug: v8:9359
Change-Id: I7b79fe619eb734343579652058be4d2b81fd4a1e
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1664060
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Peter Marshall <petermarshall@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62276}