We were decoding the names of locals into a C++ data structure, and then
generated a FixedArray out of that, stored in the on-heap WasmDebugInfo.
In order to support name lookup for debugging with Liftoff, where no
WasmDebugInfo will be present, this CL refactors the C++ data structure
to allow direct lookups and stores it in the C++ DebugInfo structure.

With this CL, the names are still only used from the old
interpreter-based debugging path. A follow-up CL will then also use it
from Liftoff.

R=thibaudm@chromium.org

Bug: v8:10019
Change-Id: I1397021b5d69b9346fc26f5e83653360f428c5e7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2002541
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Cr-Commit-Position: refs/heads/master@{#65819}