Update OSLogOptimizationc.cpp to fix warning.

Fixes the following warning:
```console
third_party/unsupported_toolchains/swift/src/swift/lib/SILOptimizer/Mandatory/OSLogOptimization.cpp:396:1: warning: control may reach end of non-void function [-Wreturn-type]
}
^
```

Author: rxwei

lib/SILOptimizer/Mandatory/OSLogOptimization.cpp

@@ -390,7 +390,7 @@ static SILValue emitCodeForSymbolicValue(SymbolicValue symVal,
     return newStructInst;
   }
   default: {
-    assert(false && "Symbolic value kind is not supported");
+    llvm_unreachable("Symbolic value kind is not supported");
   }
   }
 }