[NFC] [C++20] [Modules] Rename ASTWriter::isWritingStdCXXNamedModules

According to the discussion in
https://discourse.llvm.org/t/rfc-unifying-the-terminology-about-modules-in-clang/66054,
this patch rename ASTWriter::isWritingNamedModules to ASTWriter::isWrittingStdCXXNamedModules
to make the name more clear.

clang/include/clang/Basic/Module.h

@@ -159,7 +159,8 @@ public:
   /// eventually be exposed, for use in "private" modules.
   std::string ExportAsModule;
 
-  /// Does this Module scope describe part of the purview of a named C++ module?
+  /// Does this Module scope describe part of the purview of a standard named
+  /// C++ module?
   bool isModulePurview() const {
     return Kind == ModuleInterfaceUnit || Kind == ModulePartitionInterface ||
            Kind == ModulePartitionImplementation ||

clang/include/clang/Serialization/ASTWriter.h

@@ -736,7 +736,7 @@ public:
   bool hasChain() const { return Chain; }
   ASTReader *getChain() const { return Chain; }
 
-  bool isWritingNamedModules() const {
+  bool isWritingStdCXXNamedModules() const {
     return WritingModule && WritingModule->isModulePurview();
   }
 

clang/lib/Serialization/ASTWriter.cpp

@@ -4334,7 +4334,8 @@ void ASTRecordWriter::AddAttr(const Attr *A) {
   // FIXME: Clang can't handle the serialization/deserialization of
   // preferred_name properly now. See
   // https://github.com/llvm/llvm-project/issues/56490 for example.
-  if (!A || (isa<PreferredNameAttr>(A) && Writer->isWritingNamedModules()))
+  if (!A || (isa<PreferredNameAttr>(A) &&
+             Writer->isWritingStdCXXNamedModules()))
     return Record.push_back(0);
 
   Record.push_back(A->getKind() + 1); // FIXME: stable encoding, target attrs