From: NAKAMURA Takumi
This declares a global variable "OutputFilename" that is used to -capture the result of the "o" argument (first parameter). We specify -that this is a simple scalar option by using the "cl::opt" template (as opposed to the "cl::list template), and tell the CommandLine library +
This declares a global variable "OutputFilename" that is used to +capture the result of the "o" argument (first parameter). We specify +that this is a simple scalar option by using the "cl::opt" template (as opposed to the "cl::list template), and tell the CommandLine library that the data type that we are parsing is a string.
The second and third parameters (which are optional) are used to specify what