Compile with latest clang 7.

Changes to warnings in clang introduced by https://reviews.llvm.org/D43322
and https://reviews.llvm.org/D44883 cause warning as error failures when
building Skia. In particular this addresses return-std-move-in-c++11 and
self-assign-overloaded.

Change-Id: I680318098d8af1b64fba464585c7cdfcfcf39d66
Reviewed-on: https://skia-review.googlesource.com/123582
Reviewed-by: Greg Daniel <egdaniel@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Ben Wagner <bungeman@google.com>
4 files changed