Give a better error message when trying to run the iossim tests and...

Give a better error message when trying to run the iossim tests and `SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER` is not set in the environment.

Summary: Give a better error message when trying to run the iossim tests and `SANITIZER_IOSSIM_TEST_DEVICE_IDENTIFIER` is not set in the environment.

Reviewers: kubamracek, george.karpenkov

Subscribers: #sanitizers, llvm-commits

Differential Revision: https://reviews.llvm.org/D51272

llvm-svn: 341300