Disable SANITIZER_CHECK_DEADLOCKS on Darwin platforms.

Disable SANITIZER_CHECK_DEADLOCKS on Darwin platforms.

Although THREADLOCAL variables are supported on Darwin they cannot be
used very early on during process init (before dyld has set it up).

Unfortunately the checked lock is used before dyld has setup TLS leading
to an abort call (`_tlv_boostrap()` is never supposed to be called at
runtime).

To avoid this problem `SANITIZER_CHECK_DEADLOCKS` is now disabled on
Darwin platforms. This fixes running TSan tests (an possibly other
Sanitizers) when `COMPILER_RT_DEBUG=ON`.

For reference the crashing backtrace looks like this:

```
* thread #1, stop reason = signal SIGABRT
  * frame #0: 0x00000002044da0ae dyld`__abort_with_payload + 10
    frame #1: 0x00000002044f01af dyld`abort_with_payload_wrapper_internal + 80
    frame #2: 0x00000002044f01e1 dyld`abort_with_payload + 9
    frame #3: 0x000000010c989060 dyld_sim`abort_with_payload + 26
    frame #4: 0x000000010c94908b dyld_sim`dyld4::halt(char const*) + 375
    frame #5: 0x000000010c988f5c dyld_sim`abort + 16
    frame #6: 0x000000010c96104f dyld_sim`dyld4::APIs::_tlv_bootstrap() + 9
    frame #7: 0x000000010cd8d6d2 libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::CheckedMutex::LockImpl(this=<unavailable>, pc=<unavailable>) at sanitizer_mutex.cpp:218:58 [opt]
    frame #8: 0x000000010cd8a0f7 libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::Mutex::Lock() [inlined] __sanitizer::CheckedMutex::Lock(this=0x000000010d733c90) at sanitizer_mutex.h:124:5 [opt]
    frame #9: 0x000000010cd8a0ee libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::Mutex::Lock(this=0x000000010d733c90) at sanitizer_mutex.h:162:19 [opt]
    frame #10: 0x000000010cd8a0bf libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::GenericScopedLock<__sanitizer::Mutex>::GenericScopedLock(this=0x000000030c7479a8, mu=<unavailable>) at sanitizer_mutex.h:364:10 [opt]
    frame #11: 0x000000010cd89819 libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::GenericScopedLock<__sanitizer::Mutex>::GenericScopedLock(this=0x000000030c7479a8, mu=<unavailable>) at sanitizer_mutex.h:363:67 [opt]
    frame #12: 0x000000010cd8985b libclang_rt.tsan_iossim_dynamic.dylib`__sanitizer::LibIgnore::OnLibraryLoaded(this=0x000000010d72f480, name=0x0000000000000000) at sanitizer_libignore.cpp:39:8 [opt]
    frame #13: 0x000000010cda7aaa libclang_rt.tsan_iossim_dynamic.dylib`__tsan::InitializeLibIgnore() at tsan_interceptors_posix.cpp:219:16 [opt]
    frame #14: 0x000000010cdce0bb libclang_rt.tsan_iossim_dynamic.dylib`__tsan::Initialize(thr=0x0000000110141400) at tsan_rtl.cpp:403:3 [opt]
    frame #15: 0x000000010cda7b8e libclang_rt.tsan_iossim_dynamic.dylib`__tsan::ScopedInterceptor::ScopedInterceptor(__tsan::ThreadState*, char const*, unsigned long) [inlined] __tsan::LazyInitialize(thr=0x0000000110141400) at tsan_rtl.h:665:5 [opt]
    frame #16: 0x000000010cda7b86 libclang_rt.tsan_iossim_dynamic.dylib`__tsan::ScopedInterceptor::ScopedInterceptor(this=0x000000030c747af8, thr=0x0000000110141400, fname=<unavailable>, pc=4568918787) at tsan_interceptors_posix.cpp:247:3 [opt]
    frame #17: 0x000000010cda7bb9 libclang_rt.tsan_iossim_dynamic.dylib`__tsan::ScopedInterceptor::ScopedInterceptor(this=0x000000030c747af8, thr=<unavailable>, fname=<unavailable>, pc=<unavailable>) at tsan_interceptors_posix.cpp:246:59 [opt]
    frame #18: 0x000000010cdb72b7 libclang_rt.tsan_iossim_dynamic.dylib`::wrap_strlcpy(dst="\xd2", src="0xd1d398d1bb0a007b", size=20) at sanitizer_common_interceptors.inc:7386:3 [opt]
    frame #19: 0x0000000110542b03 libsystem_c.dylib`__guard_setup + 140
    frame #20: 0x00000001104f8ab4 libsystem_c.dylib`_libc_initializer + 65
    ...
```

rdar://83723445

Differential Revision: https://reviews.llvm.org/D111243