Port String::Flatten to Torque (using a fast C call for the
non-allocating part) and provide fast and easy access to sequential
string data in Torque: GetStringData() flattens if necessary and
computes slices that allow direct access.

Applications: String.prototype.replaceAll, String.prototype.endsWith,
  and String.prototype.beginsWith now use GetStringData() and direct
  slice access instead of the slow StringCharCodeAt and they no
  longer bail out to the runtime for flattening.

Drive-by changes:
  - Expose String instance type bits as bitfields and enums in Torque.
  - Fix method lookup in Torque to include superclass methods.
  - Use char8 and char16 types in more places.
  - Allow fast C calls with void return type.
  - Add Torque macros to create subslices.
  - Add no-GC scopes to runtime functions loading external string data.


Bug: v8:7793
Change-Id: I763b9b24212770307c9b2fe9f070f21f65d68d58
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2565515
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#71611}