Duplicate subsections in the name section are disallowed by the spec.
Since the whole name section is optional, we shouldn't fail validation
because of it, but we'll ignore duplicate subsections.

Drive-by cleanup: reduce code duplication by reusing DecodeNameMap from
DecodeIndirectNameMap.

Fixed: chromium:1342338
Change-Id: Icae14c27a0255c6107517354f07ec8eb78d2a7b1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3751211
Auto-Submit: Jakob Kummerow <jkummerow@chromium.org>
Reviewed-by: Manos Koukoutos <manoskouk@chromium.org>
Commit-Queue: Manos Koukoutos <manoskouk@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81608}