The string and view creating instructions string.new*, string.const,
string.concat, and string.as_* should all return non-nullable reference
types.

See https://github.com/WebAssembly/stringref/issues/42

Bug: v8:12868
Change-Id: I2a39aadd339a49b4aa2d145492cba85e6ab14b71
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3858236
Commit-Queue: Matthias Liedtke <mliedtke@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Matthias Liedtke <mliedtke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82792}