Change Xcode project developmentRegion to 'en' and plist CFBundleDevelopmentRegion to DEVELOPMENT_LANGUAGE (#34293)

Eliminates 64-bit hardcoding for iOS test apps under dev/. This brings
them in line with the default iOS app templates.

We now use a different approach for ios projects where the developer controls
the Xcode project file. This patch removes the old ".generated" approach in
favor of the new approach.

Initial version of complex layout performance test