Property definitions are currently defined in a PropertyDefinition array and have a corresponding enum to index in this array. Unfortunately this is quite error prone. Indeed, just today we found an incorrect merge where a discrepancy between the order of the enum values and their definition caused the test suite to fail spectacularly.
Tablegen can streamline the process of generating the property definition table while at the same time guaranteeing that the enums stay in sync. That's exactly what this patch does. It adds a new tablegen file for the properties, building on top of the infrastructure that Raphael added recently for the command options. It also introduces two new tablegen backends: one for the property definitions and one for their corresponding enums.
It might be worth mentioning that I generated most of the tablegen definitions from the existing property definitions, by adding a dump method to the struct. This seems both more efficient and less error prone that copying everything over by hand. Only Enum properties needed manual fixup for the EnumValues and DefaultEnumValue fields.