[NFC] [C++20] [Modules] Refactor Module::getGlobalModuleFragment and Module::getPrivateModuleFragment

The original implementation of `Module::getGlobalModuleFragment` and
`Module::getPrivateModuleFragment` tried to find the global module
fragment and the private module fragment by comparing strings, which
smells bad. This patch tries to improve this.

Author: ChuanqiXu9

clang/include/clang/Basic/Module.h

@@ -587,6 +587,11 @@ class alignas(8) Module {
     return Kind == ModuleInterfaceUnit || isModulePartition();
   }
 
+  /// Is this a C++20 named module unit.
+  bool isNamedModuleUnit() const {
+    return isInterfaceOrPartition() || isModuleImplementation();
+  }
+
   bool isModuleInterfaceUnit() const {
     return Kind == ModuleInterfaceUnit || Kind == ModulePartitionInterface;
   }
@@ -720,13 +725,13 @@ class alignas(8) Module {
   /// one.
   ///
   /// \returns The GMF sub-module if found, or NULL otherwise.
-  Module *getGlobalModuleFragment() { return findSubmodule("<global>"); }
+  Module *getGlobalModuleFragment() const;
 
   /// Get the Private Module Fragment (sub-module) for this module, it there is
   /// one.
   ///
   /// \returns The PMF sub-module if found, or NULL otherwise.
-  Module *getPrivateModuleFragment() { return findSubmodule("<private>"); }
+  Module *getPrivateModuleFragment() const;
 
   /// Determine whether the specified module would be visible to
   /// a lookup at the end of this module.

clang/lib/Basic/Module.cpp

@@ -370,6 +370,28 @@ Module *Module::findOrInferSubmodule(StringRef Name) {
   return Result;
 }
 
+Module *Module::getGlobalModuleFragment() const {
+  assert(isNamedModuleUnit() && "We should only query the global module "
+                                "fragment from the C++ 20 Named modules");
+
+  for (auto *SubModule : SubModules)
+    if (SubModule->isExplicitGlobalModule())
+      return SubModule;
+
+  return nullptr;
+}
+
+Module *Module::getPrivateModuleFragment() const {
+  assert(isNamedModuleUnit() && "We should only query the private module "
+                                "fragment from the C++ 20 Named modules");
+
+  for (auto *SubModule : SubModules)
+    if (SubModule->isPrivateModule())
+      return SubModule;
+
+  return nullptr;
+}
+
 void Module::getExportedModules(SmallVectorImpl<Module *> &Exported) const {
   // All non-explicit submodules are exported.
   for (std::vector<Module *>::const_iterator I = SubModules.begin(),

clang/lib/CodeGen/CodeGenModule.cpp

@@ -2915,6 +2915,9 @@ static void addLinkOptionsPostorder(CodeGenModule &CGM, Module *Mod,
 }
 
 void CodeGenModule::EmitModuleInitializers(clang::Module *Primary) {
+  assert(Primary->isNamedModuleUnit() &&
+         "We should only emit module initializers for named modules.");
+
   // Emit the initializers in the order that sub-modules appear in the
   // source, first Global Module Fragments, if present.
   if (auto GMF = Primary->getGlobalModuleFragment()) {