PIR1.5 is the RCIF (Receive Interrupt Flag).
If a byte has been received, RCIF will be set.

The USART has a 2-byte buffer, so the byte received can just sit there until the main program has time to respond.

You don't really need interrupts for the case you described, as long as the PC only sends 1-byte and waits for the PIC to reply. Then somewhere in your main loop, when the program has time ... check RCIF and respond to the request as needed.