Genex: Allow single param expressions to accept arbirary input.

Existing single-parameter expressions work due to special casing
which assumes that if there is only one parameter, and the node
accepts arbitrary content, then the result is the input.

This is true for the existing expressions matching that
pattern - namely the "1" and "TARGET_NAME" expressions. However,
the LOWER_CASE, and UPPER_CASE expressions should also accept
arbitrary content, and in their case, the result is not the input.

Refactor the cmGeneratorExpressionEvaluator to allow that extension.
Actually evaluate the "1" and "0" nodes.

Source/cmGeneratorExpressionEvaluator.cxx

@@ -82,7 +82,6 @@ static const struct ZeroNode : public cmGeneratorExpressionNode
                        const GeneratorExpressionContent *,
                        cmGeneratorExpressionDAGChecker *) const
   {
-    // Unreachable
     return std::string();
   }
 } zeroNode;
@@ -94,13 +93,12 @@ static const struct OneNode : public cmGeneratorExpressionNode
 
   virtual bool AcceptsArbitraryContentParameter() const { return true; }
 
-  std::string Evaluate(const std::vector<std::string> &,
+  std::string Evaluate(const std::vector<std::string> &parameters,
                        cmGeneratorExpressionContext *,
                        const GeneratorExpressionContent *,
                        cmGeneratorExpressionDAGChecker *) const
   {
-    // Unreachable
-    return std::string();
+    return parameters.front();
   }
 } oneNode;
 
@@ -1634,14 +1632,6 @@ std::string GeneratorExpressionContent::Evaluate(
     return std::string();
     }
 
-  if (node->NumExpectedParameters() == 1
-        && node->AcceptsArbitraryContentParameter())
-    {
-    return this->ProcessArbitraryContent(node, identifier, context,
-                                         dagChecker,
-                                         this->ParamChildren.begin());
-    }
-
   std::vector<std::string> parameters;
   this->EvaluateParameters(node, identifier, context, dagChecker, parameters);
   if (context->HadError)
@@ -1669,33 +1659,35 @@ std::string GeneratorExpressionContent::EvaluateParameters(
                                         pend = this->ParamChildren.end();
   const bool acceptsArbitraryContent
                                   = node->AcceptsArbitraryContentParameter();
-  for ( ; pit != pend; ++pit)
+  int counter = 1;
+  for ( ; pit != pend; ++pit, ++counter)
     {
-    std::string parameter;
-    std::vector<cmGeneratorExpressionEvaluator*>::const_iterator it =
-                                                              pit->begin();
-    const std::vector<cmGeneratorExpressionEvaluator*>::const_iterator end =
-                                                              pit->end();
-    for ( ; it != end; ++it)
-      {
-      parameter += (*it)->Evaluate(context, dagChecker);
-      if (context->HadError)
-        {
-        return std::string();
-        }
-      }
-    parameters.push_back(parameter);
-    if (acceptsArbitraryContent
-        && parameters.size() == (unsigned int)numExpected - 1)
+    if (acceptsArbitraryContent && counter == numExpected)
       {
-      assert(pit != pend);
       std::string lastParam = this->ProcessArbitraryContent(node, identifier,
                                                             context,
                                                             dagChecker,
-                                                            pit + 1);
+                                                            pit);
       parameters.push_back(lastParam);
       return std::string();
       }
+    else
+      {
+      std::string parameter;
+      std::vector<cmGeneratorExpressionEvaluator*>::const_iterator it =
+                                                                pit->begin();
+      const std::vector<cmGeneratorExpressionEvaluator*>::const_iterator end =
+                                                                pit->end();
+      for ( ; it != end; ++it)
+        {
+        parameter += (*it)->Evaluate(context, dagChecker);
+        if (context->HadError)
+          {
+          return std::string();
+          }
+        }
+      parameters.push_back(parameter);
+      }
     }
   }