cmSourceFile: Remove non-const GetProperties accessor

Manipulating the property map of cmSourceFile directly may invalidate the class
invariant of the cached value IsGenerated.  Provide the setter SetProperties
which also updates IsGenerated.

Source/cmCPluginAPI.cxx

@@ -561,7 +561,7 @@ void* CCONV cmAddSource(void* arg, void* arg2)
 
   // Create the real cmSourceFile instance and copy over saved information.
   cmSourceFile* rsf = mf->GetOrCreateSource(osf->FullPath);
-  rsf->GetProperties() = osf->Properties;
+  rsf->SetProperties(osf->Properties);
   for (std::string const& d : osf->Depends) {
     rsf->AddDepend(d);
   }

Source/cmSourceFile.cxx

@@ -314,6 +314,13 @@ bool cmSourceFile::GetPropertyAsBool(const std::string& prop) const
   return cmIsOn(this->GetProperty(prop));
 }
 
+void cmSourceFile::SetProperties(cmPropertyMap properties)
+{
+  this->Properties = std::move(properties);
+
+  this->IsGenerated = this->GetPropertyAsBool(propGENERATED);
+}
+
 cmCustomCommand* cmSourceFile::GetCustomCommand() const
 {
   return this->CustomCommand.get();

Source/cmSourceFile.h

@@ -94,8 +94,9 @@ public:
   void AddDepend(const std::string& d) { this->Depends.push_back(d); }
 
   // Get the properties
-  cmPropertyMap& GetProperties() { return this->Properties; }
   const cmPropertyMap& GetProperties() const { return this->Properties; }
+  // Set the properties
+  void SetProperties(cmPropertyMap properties);
 
   /**
    * Check whether the given source file location could refer to this