[CMake][NFC] Refactor iOS simulator/device test configuration generation code for ASan.
The previous code hard-coded platform names but compiler-rt's CMake
build system actually already knows which Apple platforms ASan supports.
This change uses this information to enumerate the different Apple
Reviewers: kubamracek, yln
Subscribers: mgorny, #sanitizers, llvm-commits
Tags: #sanitizers, #llvm
Differential Revision: https://reviews.llvm.org/D73232