cmSourceFile: Add IsGenerated method

All cmSourceFiles are checked at least once whether they're `GENERATED` or not.
This adds a convenience method `GetIsGenerated` that returns a private
boolean cache variable `IsGenerated`.  `IsGenerated` is updated every time the
`GENERATED` property is written.

Source/cmSourceFile.cxx

@@ -17,8 +17,6 @@ cmSourceFile::cmSourceFile(cmMakefile* mf, const std::string& name,
                            cmSourceFileLocationKind kind)
   : Location(mf, name, kind)
 {
-  this->CustomCommand = nullptr;
-  this->FindFullPathFailed = false;
 }
 
 cmSourceFile::~cmSourceFile()
@@ -244,12 +242,22 @@ bool cmSourceFile::Matches(cmSourceFileLocation const& loc)
 void cmSourceFile::SetProperty(const std::string& prop, const char* value)
 {
   this->Properties.SetProperty(prop, value);
+
+  // Update IsGenerated flag
+  if (prop == propGENERATED) {
+    this->IsGenerated = cmSystemTools::IsOn(value);
+  }
 }
 
 void cmSourceFile::AppendProperty(const std::string& prop, const char* value,
                                   bool asString)
 {
   this->Properties.AppendProperty(prop, value, asString);
+
+  // Update IsGenerated flag
+  if (prop == propGENERATED) {
+    this->IsGenerated = this->GetPropertyAsBool(propGENERATED);
+  }
 }
 
 const char* cmSourceFile::GetPropertyForUser(const std::string& prop)

Source/cmSourceFile.h

@@ -55,6 +55,10 @@ public:
       command like get_property or get_source_file_property.  */
   const char* GetPropertyForUser(const std::string& prop);
 
+  ///! Checks is the GENERATED property is set and true
+  /// @return Equivalent to GetPropertyAsBool("GENERATED")
+  bool GetIsGenerated() const { return this->IsGenerated; }
+
   /**
    * The full path to the file.  The non-const version of this method
    * may attempt to locate the file on disk and finalize its location.
@@ -106,13 +110,14 @@ public:
 private:
   cmSourceFileLocation Location;
   cmPropertyMap Properties;
-  cmCustomCommand* CustomCommand;
+  cmCustomCommand* CustomCommand = nullptr;
   std::string Extension;
   std::string Language;
   std::string FullPath;
   std::string ObjectLibrary;
   std::vector<std::string> Depends;
-  bool FindFullPathFailed;
+  bool FindFullPathFailed = false;
+  bool IsGenerated = false;
 
   bool FindFullPath(std::string* error);
   bool TryFullPath(const std::string& path, const std::string& ext);