If we create OBJECTS_DIR at qmake time, Xcode will not consider this
directory as created by the build system, and "xcodebuild --clean" will
fail.
Prevent qmake from creating that directory in the Xcode generator.
Pick-to: 5.15 6.2
Fixes: QTBUG-96305
Change-Id: I874bf34a4289ce5f2d3e2ce070ffbe56d5368861
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>