[clang] [clang][driver][darwin] Report bad SDKSettings as a fatal error rather than unreachable (PR #175073)

[Ian Anderson via cfe-commits]

https://github.com/ian-twilightcoder created https://github.com/llvm/llvm-project/pull/175073

Fatal error is more appropriate than unreachable when the SDKSettings is not in a recognized form (encountered in a few tests with incomplete SDKSettings.json).

>From cf3709ed2193df84c50a3a8e2d62b81468f878d0 Mon Sep 17 00:00:00 2001
From: Ian Anderson <iana at apple.com>
Date: Thu, 8 Jan 2026 13:32:37 -0800
Subject: [PATCH] [clang][driver][darwin] Report bad SDKSettings as a fatal
 error rather than unreachable

Fatal error is more appropriate than unreachable when the SDKSettings is not in a recognized form (encountered in a few tests with incomplete SDKSettings.json).
---
 clang/lib/Basic/DarwinSDKInfo.cpp | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/clang/lib/Basic/DarwinSDKInfo.cpp b/clang/lib/Basic/DarwinSDKInfo.cpp
index 4fd31b841b23d..b55ffd1c39a86 100644
--- a/clang/lib/Basic/DarwinSDKInfo.cpp
+++ b/clang/lib/Basic/DarwinSDKInfo.cpp
@@ -112,7 +112,9 @@ parsePlatformInfos(const llvm::json::Object &Obj, VersionTuple Version) {
                                llvm::Triple::UnknownEnvironment,
                                llvm::Triple::MachO, "/System/DriverKit"});
     } else {
-      llvm_unreachable("Unrecognized Xcode platform");
+      llvm::reportFatalUsageError(
+          "Unrecognized CanonicalName in SDKSettings.json. SupportedTargets is "
+          "expected, or a recognized CanonicalName.");
     }
     return PlatformInfos;
   }