Replace obsolete clang option
Replace obsolete clang option

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 <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Reviewed-by: Nico Weber <c0f02d98506190746d1cdcb01be0f37b51e27077@chromium.org>
Reviewed-by: Jamie Madill <7e492b4f1c8458024932de3ba475cbf015424c30@chromium.org>

ppy/angle,ppy/angle,ppy/angle,ppy/angle