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

[Cyndy Ishida via cfe-commits]

================
@@ -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.
----------------
cyndyishida wrote:

```suggestion
  /// Information about the supported platforms, derived from the target triple definitions, in the SDK.
```

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