Fix MSVC "not all control paths return a value" warning. NFCI.

Author: RKSimon

clang/lib/Driver/ToolChains/Darwin.cpp

@@ -1932,6 +1932,7 @@ struct DarwinPlatform {
     case DarwinPlatformKind::XROS:
       return llvm::Triple::XROS;
     }
+    llvm_unreachable("Unknown DarwinPlatformKind enum");
   }
 
   SourceKind Kind;