Force lit to execute the ASan and TSan tests on iOS devices

sequentially.

The current implementation of commands in
`test/sanitizer_common/ios_commands/` for iOS devices cannot be executed
in parallel which results in the ASan and TSan tests failing when
executed in parallel by lit which was the default behaviour.

We now force the ASan and TSan tests to be a new parallelism group named
`darwin-ios-device-sanitizer` which allows only one test to be run at a
time.  We also emit a warning informing the user that tests are being
run sequentially.

This only applies if the target is an iOS device.

Differential Revision: https://reviews.llvm.org/D42156

llvm-svn: 323026