StringConstantXXX were introduced when we switched to concurrent
compilation, as a way to build strings in Turbofan in a background
thread, without having to actually allocate them on the main heap
from the background. See https://crrev.com/c/1221807.

Now that we have local heaps, we can actually allocate strings from
the background, making StringConstantXXX useless.

Moreover, we would fold constant string concatenations into
ConsString, which sounds a bit dubious for performance. Now, small
constant string concatenations will be folded into SeqStrings, while
larger ones will remain ConsString, just to avoid the quadratic
worst-case.

Change-Id: I0479d16aa5691c9d774187c4cc0d03ff4fe2b4f9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3811291Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Darius Mercadier <dmercadier@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82381}