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

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.

Differential Revision: https://reviews.llvm.org/D120764

clang/include/clang/Basic/Module.h

@@ -515,12 +515,14 @@ class Module {
   }
 
   /// 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);
     }