Moves the following functions from CodeStubAssembler to
BuiltinsStringAssembler:

SubString
CopyStringCharacters
AllocAndCopyStringCharacters

Bug: v8:9396
Change-Id: Ieb534b7fa7e72db9b05cdc2a34bd88b7a52ee985
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1822040Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63959}