Merge remote-tracking branch 'upstream/3.4' into merge-3.4

diff --cc CMakeLists.txt
Simple merge

diff --cc cmake/OpenCVCompilerOptions.cmake
Simple merge

diff --cc modules/dnn/src/onnx/onnx_graph_simplifier.cpp
Simple merge

diff --cc modules/dnn/test/test_onnx_importer.cpp
Simple merge

diff --cc platforms/ios/cmake/Toolchains/common-ios-toolchain.cmake
Simple merge