[C++20][Modules] Improve efficiency of isModuleParition method.

[iains]

Summary

The original implementation of this used the presence of a ":" in the module
name as the key, but since we now generate modules with the correct kind, we
can just test that.

[iains]

This should improve the efficiency in the demangler use-case.

[ChuanqiXu]

LGTM.

clang/include/clang/Basic/Module.h

public:
   void setParent(Module *M) {
     assert(!Parent);
     Parent = M;
     Parent->SubModuleIndex[Name] = Parent->SubModules.size();
     Parent->SubModules.push_back(this);
   }
 
   /// Is this a module partition.
-  bool isModulePartition() const { return Name.find(':') != std::string::npos; }
+  bool isModulePartition() const {
+    return Kind == ModulePartitionInterface ||
+           Kind == ModulePartitionImplementation;
+  }
 
   /// Get the primary module interface name from a partition.
   StringRef getPrimaryModuleInterfaceName() const {
-    if (Kind == ModulePartitionInterface ||
-        Kind == ModulePartitionImplementation) {
+    if (isModulePartition()) {
       auto pos = Name.find(':');
       return StringRef(Name.data(), pos);
     }
     return Name;
   }
 
   /// Retrieve the full name of this module, including the path from
   /// its top-level module.