Various TurboFan operators and builtins currently conflate the concept of a Unicode code point with that of its UTF-32 and UTF-16 encoding. UTF-16 is only used as an internal optimization, and should not be exposed. This CL separates affected operators and clarifies the naming of various internals.

Prior to this CL, StringCodePointAt operator's typing rule was unsound, since it was not strictly limited to returning values in the range of Unicode code points.

R=sigurds@chromium.org

Bug: v8:9413
Change-Id: Icd32fdbeceadbc74e9761b717a27e3ff9ecde1f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1683998
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Reviewed-by: Benedikt Meurer <bmeurer@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#62490}