[Support] Update comment to match actual behavior.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@263848 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/Error.h

@@ -163,8 +163,7 @@ public:
   /// considered unchecked. The source error becomes a checked success value,
   /// regardless of its original state.
   Error &operator=(Error &&Other) {
-    // Move assignment shouldn't drop an existing error, but we won't complain
-    // about overwriting success.
+    // Don't allow overwriting of unchecked values.
     assertIsChecked();
     setPtr(Other.getPtr());