[NFCI] [Serialization] Use demoteThisDefinitionToDeclaration instead of setCompleteDefinition(false) for CXXRecordDecl

When we merge the definition for CXXRecordDecl, we would use
setCompleteDefinition(false) to mark the merged definition. But this was
not the correct/good interface. We can't know that the merged definition
was a definition then. And actually, we provided an interface for this:
demoteThisDefinitionToDeclaration.

So this patch tries to use the correct API.

This was found in the downstream developing. This is not strictly NFC
but it is intended to be NFC for every end users.

Author: ChuanqiXu9

clang/lib/Serialization/ASTReaderDecl.cpp

@@ -2064,7 +2064,7 @@ void ASTDeclMerger::MergeDefinitionData(
     Reader.MergedDeclContexts.insert(std::make_pair(MergeDD.Definition,
                                                     DD.Definition));
     Reader.PendingDefinitions.erase(MergeDD.Definition);
-    MergeDD.Definition->setCompleteDefinition(false);
+    MergeDD.Definition->demoteThisDefinitionToDeclaration();
     Reader.mergeDefinitionVisibility(DD.Definition, MergeDD.Definition);
     assert(!Reader.Lookups.contains(MergeDD.Definition) &&
            "already loaded pending lookups for merged definition");
@@ -2175,6 +2175,9 @@ void ASTDeclReader::ReadCXXRecordDefinition(CXXRecordDecl *D, bool Update,
   D->DefinitionData = Canon->DefinitionData;
   ReadCXXDefinitionData(*DD, D, LambdaContext, IndexInLambdaContext);
 
+  // Mark this declaration as being a definition.
+  D->setCompleteDefinition(true);
+
   // We might already have a different definition for this record. This can
   // happen either because we're reading an update record, or because we've
   // already done some merging. Either way, just merge into it.
@@ -2183,9 +2186,6 @@ void ASTDeclReader::ReadCXXRecordDefinition(CXXRecordDecl *D, bool Update,
     return;
   }
 
-  // Mark this declaration as being a definition.
-  D->setCompleteDefinition(true);
-
   // If this is not the first declaration or is an update record, we can have
   // other redeclarations already. Make a note that we need to propagate the
   // DefinitionData pointer onto them.