[clang] [llvm] [clang][driver][darwin] Base the system prefix on the SDK, not the target (PR #171970)

[Ian Anderson via llvm-commits]

ian-twilightcoder wrote:

I'm tempted to make the system prefix finding a little more generous.
1. Don't match on arch, call it a match if the triple matches vendor-os-env
2. Fall back on the default SupportedTarget's `SystemPrefix` if there isn't a match
Any opinions?

https://github.com/llvm/llvm-project/pull/171970