Translator: Fix pruning functions that declare a struct When pruning unused functions, the entire declaration of the function is removed. If the function declares a struct as part of its return value, the declaration of that struct was lost. This change makes sure that declaration survives the pruning. For example: struct S { vec4 v; } unused_func() { ... } is now replaced with: struct S { vec4 v; }; This change also makes struct usage validation more stringent. Bug: chromium:1248753 Change-Id: Idd9a87fd2e785135775cfea62995fd33adaf3c09 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/3169691 Reviewed-by: Tim Van Patten <timvp@google.com> Reviewed-by: Jamie Madill <jmadill@chromium.org> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>