[lldb] Fix a regression in SBValue::GetObjectDescription() (#117242)

The old behavior was to return a null string in the error case,when
refactoring the error handling I thought it would be a good idea to
print the error in the description, but that breaks clients that try to
print a description first and then do something else in the error case.
The API is not great but it's clear that in-band errors are also not a
good idea.

rdar://133956263

lldb/source/API/SBValue.cpp

@@ -380,8 +380,10 @@ const char *SBValue::GetObjectDescription() {
     return nullptr;
 
   llvm::Expected<std::string> str = value_sp->GetObjectDescription();
-  if (!str)
-    return ConstString("error: " + toString(str.takeError())).AsCString();
+  if (!str) {
+    llvm::consumeError(str.takeError());
+    return nullptr;
+  }
   return ConstString(*str).AsCString();
 }
 

lldb/test/API/commands/expression/diagnostics/TestExprDiagnostics.py

@@ -195,6 +195,10 @@ note: candidate function not viable: requires single argument 'x', but 2 argumen
         value = frame.EvaluateExpression('#error("I am error.")')
         error = value.GetError()
         self.assertEqual(error.GetType(), lldb.eErrorTypeExpression)
+        value = frame.FindVariable("f")
+        self.assertTrue(value.IsValid())
+        desc = value.GetObjectDescription()
+        self.assertEqual(desc, None)
 
     def test_command_expr_sbdata(self):
         """Test the structured diagnostics data"""