Moves the following functions only used in string builtins out of
CodeStubAssembler:

StringAdd
AllocateConsString
StringFromSingleUTF16EncodedCodePoint
BranchIfCanDerefIndirectString
DerefIndirectString
MaybeDerefIndirectString
MaybeDerefIndirectStrings

Bug: v8:9396
Change-Id: Ib89966b9c170ca23dd7535a0f550c69966a6e21c
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1817608
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63932}