Fix PackedVarying's move constructor The move constructor was calling the move assignment operator, which used std::swap on the members. However, `this` was not constructed, so std::swap was operating on uninitialized values. Caught by UBSAN. Bug: chromium:1159444 Change-Id: I65f2f0affd1461f880c59780f31a368779411f24 Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2599936 Reviewed-by: Jamie Madill <jmadill@chromium.org> Reviewed-by: Ian Elliott <ianelliott@google.com> Commit-Queue: Shahbaz Youssefi <syoussefi@chromium.org>