Predicate variadicFormatter

Holds if f is a function such as vprintf that has a format parameter and a variable argument list of type va_arg. formatParamIndex indicates the format parameter and type indicates the type of vprintf:

• "" is a vprintf variant, outputParamIndex is -1.
• "f" is a vfprintf variant, outputParamIndex indicates the output stream parameter.
• "s" is a vsprintf variant, outputParamIndex indicates the output buffer parameter.
• "?" if the type cannot be determined. outputParamIndex is -1.