This is essentially a follow up to https://reviews.llvm.org/D155072
This adds support for also passing properties as `ref` parameter to `custom`. This requires the property to have been bound previously and will error otherwise. This makes it possible for an implementation of `custom` to take previously parsed data into account, creating nice context-dependent grammars :-)
Differential Revision: https://reviews.llvm.org/D155297
f69a9f3974