Port de04959f

Original Commit Message:

    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.

R=dmercadier@chromium.org, joransiu@ca.ibm.com, junyan@redhat.com, midawson@redhat.com
BUG=
LOG=N

Change-Id: I0fb010dfda4a94ba8c5d09de96fa1932c6c91113
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3826049Reviewed-by: Darius Mercadier <dmercadier@chromium.org>
Commit-Queue: Milad Farazmand <mfarazma@redhat.com>
Cr-Commit-Position: refs/heads/main@{#82416}