Convert: Remove last use of START enum value

The enums just introduce unfamiliar names and don't provide value.

The order of arguments is reversed, as that is the result of the

 cmOutputConverter::ConvertToRelativePath

overload which accepts the RelativeRoot enum.

Source/cmLocalGenerator.cxx

@@ -2514,7 +2514,8 @@ std::string cmLocalGenerator::GetObjectFileNameWithoutTarget(
   const char* fullPath = source.GetFullPath().c_str();
 
   // Try referencing the source relative to the source tree.
-  std::string relFromSource = this->ConvertToRelativePath(fullPath, START);
+  std::string relFromSource =
+    this->ConvertToRelativePath(this->GetCurrentSourceDirectory(), fullPath);
   assert(!relFromSource.empty());
   bool relSource = !cmSystemTools::FileIsFullPath(relFromSource.c_str());
   bool subSource = relSource && relFromSource[0] != '.';