Cannot get HC-12 to function with Arduino Pro Micro clone
Solution to problem is as follows:
 For the Arduino Pro Micro [as for the Leonardo], Serial1 must be used instead of a SoftwareSerial specification, e.g., 

SoftwareSerial HC12(HC12TxdPin, HC12RxdPin).


Code is to be modified by changing HC12.available() to Serial1.available().  The same holds true for the reads and writes.
Pins are to be wired such that the HC-12 RXD pin connects to the Pro Micro TXD pin, and the HC-12 TXD pin connects to the Pro Micro RXD pin. 
My thanks to @mico for providing the reference that explains it all.