Rename `ExistsCheck.to_string` to reflect its intent as a debugging utility
Rename `ExistsCheck.to_string` to reflect its intent as a debugging utility

Reviewed By: dsainati1

Differential Revision: D10854252

fbshipit-source-id: da490d69de44a7eaa555e69597f0e4bb1f62a393

AgentME/flow,gabelevi/flow,facebook/flow,facebook/flow,popham/flow,popham/flow,AgentME/flow,popham/flow,AgentME/flow,mroch/flow,samwgoldman/flow,gabelevi/flow,jamesgpearce/flow,gabelevi/flow,popham/flow,facebook/flow,mroch/flow,popham/flow,jamesgpearce/flow,facebook/flow,gabelevi/flow,facebook/flow,samwgoldman/flow,popham/flow,mroch/flow,popham/flow,gabelevi/flow,gabelevi/flow,jamesgpearce/flow,samwgoldman/flow,mroch/flow,samwgoldman/flow,samwgoldman/flow,jamesgpearce/flow,jamesgpearce/flow,AgentME/flow,gabelevi/flow,jamesgpearce/flow,mroch/flow,mroch/flow,facebook/flow,jamesgpearce/flow,samwgoldman/flow,samwgoldman/flow,facebook/flow,AgentME/flow,mroch/flow