Rename StringShape::full_representation_tag to
StringShape::representation_and_encoding_tag, since the full
representation tag now includes the shared bit.

There are no users of the new method in this CL; this is split out to
make subsequent shared string CLs smaller.

Bug: v8:12007
Change-Id: Ic4ac0241fd9846241e85b4a094dfee6d201ba42b
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3313428Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Reviewed-by: Patrick Thier <pthier@chromium.org>
Commit-Queue: Shu-yu Guo <syg@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78253}