cmGeneratorExpression: Add comments on implementation details

Clarify the purpose of some genex code that collects properties over the
transitive link closure.

Source/cmGeneratorExpressionDAGChecker.cxx

@@ -141,12 +141,15 @@ bool cmGeneratorExpressionDAGChecker::GetTransitivePropertiesOnlyCMP0131()
 
 bool cmGeneratorExpressionDAGChecker::EvaluatingGenexExpression() const
 {
+  // Corresponds to GenexEvaluator::EvaluateExpression.
   return cmHasLiteralPrefix(this->Property, "TARGET_GENEX_EVAL:") ||
     cmHasLiteralPrefix(this->Property, "GENEX_EVAL:");
 }
 
 bool cmGeneratorExpressionDAGChecker::EvaluatingPICExpression() const
 {
+  // Corresponds to checkInterfacePropertyCompatibility's special case
+  // that evaluates the value of POSITION_INDEPENDENT_CODE as a genex.
   return this->Top->Property == "INTERFACE_POSITION_INDEPENDENT_CODE";
 }
 

Source/cmGeneratorExpressionNode.cxx

@@ -547,6 +547,7 @@ static const struct TargetGenexEvalNode : public GenexEvaluator
       return expression;
     }
 
+    // Replace the surrounding context with the named target.
     cmGeneratorExpressionContext targetContext(
       context->LG, context->Config, context->Quiet, target, target,
       context->EvaluateForBuildsystem, context->Backtrace, context->Language);
@@ -2897,6 +2898,9 @@ static const struct TargetPropertyNode : public cmGeneratorExpressionNode
     bool evaluatingLinkLibraries = false;
 
     if (dagCheckerParent) {
+      // This $<TARGET_PROPERTY:...> node has been reached while evaluating
+      // another target property value.  Check that the outermost evaluation
+      // expects such nested evaluations.
       if (dagCheckerParent->EvaluatingGenexExpression() ||
           dagCheckerParent->EvaluatingPICExpression() ||
           dagCheckerParent->EvaluatingLinkerLauncher()) {

Source/cmGeneratorTarget.cxx

@@ -6375,6 +6375,7 @@ PropertyType checkInterfacePropertyCompatibility(cmGeneratorTarget const* tgt,
   std::string interfaceProperty = "INTERFACE_" + p;
   std::unique_ptr<cmGeneratorExpressionInterpreter> genexInterpreter;
   if (p == "POSITION_INDEPENDENT_CODE") {
+    // Corresponds to EvaluatingPICExpression.
     genexInterpreter = cm::make_unique<cmGeneratorExpressionInterpreter>(
       tgt->GetLocalGenerator(), config, tgt);
   }