[clang] Infer visionOS availability from iOS availability (PR #176990)

[LLVM Continuous Integration via cfe-commits]

llvm-ci wrote:

LLVM Buildbot has detected a new failure on builder `arc-builder` running on `arc-worker` while building `clang` at step 6 "test-build-unified-tree-check-all".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/3/builds/27622

<details>
<summary>Here is the relevant piece of the build log for the reference</summary>

```
Step 6 (test-build-unified-tree-check-all) failure: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
...
8.279 [38/18/16] Linking CXX executable unittests/CGData/CGDataTests
8.426 [37/18/17] Linking CXX executable bin/OrcV2CBindingsRemovableCode
8.900 [36/18/18] Linking CXX executable unittests/Bitcode/BitcodeTests
9.187 [35/18/19] Linking CXX executable bin/Kaleidoscope-Ch6
9.388 [34/18/20] Linking CXX executable tools/lld/unittests/AsLibELF/LLDAsLibELFTests
9.558 [33/18/21] Linking CXX executable bin/Kaleidoscope-Ch5
9.624 [32/18/22] Linking CXX executable unittests/DebugInfo/BTF/DebugInfoBTFTests
9.723 [31/18/23] Linking CXX executable unittests/DebugInfo/GSYM/DebugInfoGSYMTests
9.794 [30/18/24] Linking CXX executable tools/lld/unittests/AsLibAll/LLDAsLibAllTests
10.035 [29/18/25] Linking CXX executable bin/OrcV2CBindingsLazy
command timed out: 1200 seconds without output running [b'ninja', b'check-all'], attempting to kill
process killed by signal 9
program finished with exit code -1
elapsedTime=1210.841463

```

</details>

https://github.com/llvm/llvm-project/pull/176990