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

Summary:
The previous code hard-coded platform names but compiler-rt's CMake
build system actually already knows which Apple platforms TSan supports.

This change uses this information to enumerate the different Apple
platforms.

This change relies on the `get_capitalized_apple_platform()` function
added in a previous commit.

rdar://problem/58798733

Reviewers: kubamracek, yln

Subscribers: mgorny, #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D73238