This is a reland of commit 07e11a64.

The original change removed the fill_thehole_and_call_runtime bailout
in StringBuiltinsAssembler::StringToArray() so when the string
is external and cannot be unpacked, the FixedArray won't be filled
with holes before we call into the runtime, thus failing a
heap verification if a GC happens before the array is filled. This
reland adds back the bailout for this case.

Bug: v8:12718, chromium:1330410

Original change's description:
> [heap] pre-populate the single_character_string_cache
>
> This simplifies the code and removes the runtime overhead of
> spontaneously adding strings to the cache.
>
> Bug: v8:12718
> Change-Id: I2ed49bd82e3baf2563eeb8f463be72c0308c52c5
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3616553
> Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
> Reviewed-by: Leszek Swirski <leszeks@chromium.org>
> Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
> Commit-Queue: Joyee Cheung <joyee@igalia.com>
> Cr-Commit-Position: refs/heads/main@{#80803}

Change-Id: I25e8724d511a8d0d971fa2a9b6ba8a0eafce4413
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3793525Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Joyee Cheung <joyee@igalia.com>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82082}