Clang has retired `-Wreturn-std-move-in-c++11`, which was a diagnostic
to help ensure backward compatibility with old versions of Clang.

Clang still has a similar-but-different `-Wreturn-std-move` that helps
find copies-on-return that can be changed to moves-on-return.

Bug: angleproject:5657
Change-Id: I4eaf9fb9eb8b4d7f9823025549e1267af2324f3f
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2702754
Commit-Queue: Nico Weber <thakis@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>