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

[Ian Anderson via llvm-commits]

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@@ -181,6 +192,7 @@ class DarwinSDKInfo {
   VersionTuple Version;
   VersionTuple MaximumDeploymentTarget;
   llvm::Triple::OSType OS;
+  std::unordered_map<llvm::Triple, std::string> SystemPrefixes;
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ian-twilightcoder wrote:

I'm not really sure if this makes better sense as a `SmallDenseMap` or an `unordered_map`. I tried both (the first commit uses `SmallDenseMap` and the second replaces it with `unordered_map`) and they both work. `unordered_map` is a little less code in `Triple` to make it work, and @felipepiovezan suggested that was better because `Triple` might be a bit large to efficiently use as a `SmallDenseMap` key.

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