[C++20] [Modules] Don't emit macro definitions with named module

It is meaningless to emit macro definitions for named modules. With some
small experiments, the size of the module for the named modules reduced
2%~4% after this patch.

clang/lib/Serialization/ASTWriter.cpp

@@ -2318,11 +2318,14 @@ void ASTWriter::WritePreprocessor(const Preprocessor &PP, bool IsModule) {
   // Construct the list of identifiers with macro directives that need to be
   // serialized.
   SmallVector<const IdentifierInfo *, 128> MacroIdentifiers;
-  for (auto &Id : PP.getIdentifierTable())
-    if (Id.second->hadMacroDefinition() &&
-        (!Id.second->isFromAST() ||
-         Id.second->hasChangedSinceDeserialization()))
-      MacroIdentifiers.push_back(Id.second);
+  // It is meaningless to emit macros for named modules. It only wastes times
+  // and spaces.
+  if (!isWritingStdCXXNamedModules())
+    for (auto &Id : PP.getIdentifierTable())
+      if (Id.second->hadMacroDefinition() &&
+          (!Id.second->isFromAST() ||
+          Id.second->hasChangedSinceDeserialization()))
+        MacroIdentifiers.push_back(Id.second);
   // Sort the set of macro definitions that need to be serialized by the
   // name of the macro, to provide a stable ordering.
   llvm::sort(MacroIdentifiers, llvm::deref<std::less<>>());

clang/test/Modules/cxx20-module-file-info-macros.cpp

@@ -62,13 +62,4 @@ module;
 #include "foo.h"
 export module M;
 #define M_Module 43
-// FIXME: It is meaningless for named modules to emit macro definitions.
-// It wastes the time and spaces completely.
-// CHECK: Macro Definitions:
-// CHECK-DAG: M_Module
-// CHECK-DAG: REDEFINE
-// CHECK-DAG: FUNC_Macro
-// CHECK-DAG: TO_BE_UNDEF
-// CHECK-DAG: FOO
-// CHECK-DAG: CONSTANT
-// CHECK-NEXT: ===
+// CHECK-NOT: Macro Definitions: