[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.