[Sanitizer] Disable LSan when testing on iOS simulators

LSan (`ASAN_OPTIONS=detect_leaks=1`) is supported on macOS, but disabled
by default on Darwin (`SANITIZER_MAC`):
```
COMMON_FLAG(bool, detect_leaks, !SANITIZER_MAC, "Enable memory leak detection.")
```

We enable it here for ASan tests to prevent regressions (per comment).
However, LSan is not supported for the iOS simulator and the tests fail
when it is enabled.

Make this "Is macOS?" check more precise since the current one (`Darwin
&& x86_64`) has two issues:
 * Includes the simulators
 * Excludes macOS on Apple Silicon

This will allow us to (re)enable simulator testing on Green dragon to
give open source better feedback about sanitizer changes:
https://green.lab.llvm.org

rdar://86529234

Differential Revision: https://reviews.llvm.org/D115816