This won't affect device tests on the bots (because those already set LLVM
manually when testing Release), but it becomes less confusing when trying to
reproduce any problems locally, since now the project configuration on disk
matches the tested configuration.
Don't *always* codesign, especially for iOS8 which seems to break.
iOS Simulator builds should only be codesigned if they require
Entitlements (signified by RequireProvisionProfile).