[asan] Add support for running lit tests in the iOS Simulator

This patch adds a basic support for running the ASan lit test suite against an iOS Simulator. This is done by generating more lit.site.cfg configurations into subdirectories such as IOSSimI386Config and IOSSimX86_64Config. These test suites are not added into "check-all" or into "check-asan", they have to be run manually.

Differential Revision: https://reviews.llvm.org/D31477

llvm-svn: 301443