... and refactor String.prototype.substring.

This is done to simplify cleaning up callers of ConvertToRelativeIndex()
in a follow-up CL.

This CL also introduces Smi-overflow friendly helper function
  ClampToIndexRange(index: JSAny, min: uintptr, max: uintptr): uintptr

which can be used in other String builtins as a better alternative to
  NumberMin(NumberMax(value, min), max)
pattern.

Bug: v8:8996, v8:4153
Change-Id: Ie1bb5ab305ebf851c033d109ffe9e6afb9418274
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1872392
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64463}