[CMake][NFC] Refactor iOS simulator/device test configuration generation code for ASan.

Summary:
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
platforms.

rdar://problem/58798733

Reviewers: kubamracek, yln

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73232