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

[Ian Anderson via cfe-commits]

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@@ -28,6 +32,38 @@ namespace clang {
 /// The information about the darwin SDK that was used during this compilation.
 class DarwinSDKInfo {
 public:
+  /// Information about the supported platforms in the SDK
+  struct SupportedPlatform {
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ian-twilightcoder wrote:

Is `SupportedTarget` any better? I was going to call it that at first since that's the key it comes from but is it too confusing with "target triple"? 

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