[clang] [clang] [ARM] Explicitly enable NEON for Windows/Darwin targets (PR #122095)

[David Spickett via cfe-commits]

https://github.com/DavidSpickett approved this pull request.

My understanding is that:
* This PR does not conflict with what the Debian patch does.
* It in fact will fix a bug in the Debian build.
* This does not change the behaviour of an unmodified Clang from a user's perspective, therefore we do not need to release note this.

On that basis, LGTM.

https://github.com/llvm/llvm-project/pull/122095