[clang] [Clang] Reject __annotation on unsupported targets (PR #193731)

[via cfe-commits]

kjedruczyk wrote:

Hmm, looks like an msan test failed on AArch64:

```
  FAIL: SanitizerCommon-msan-aarch64-Linux :: Linux/pthread_join.cpp (3425 of 5175)
  ******************** TEST 'SanitizerCommon-msan-aarch64-Linux :: Linux/pthread_join.cpp' FAILED ********************
  Exit Code: 1
```

Doesn't strike me as related to my change - are, by any chance, any of these tests known to be unreliable?

https://github.com/llvm/llvm-project/pull/193731