Where is the text for the symbol stored in LTspice?
It's stored in the .asc file that contains your circuit. This makes sense because you don't want to store the name for a specific part in the part file, rather anything you rename a part(C1, C2, C3, Etc) gets stored in the .asc file which contains the components and wiring of your circuit. 

Edit: I understand now what you were asking. The line SYMATTR Prefix X is the line that the U is stored under. I don't know why the character X shows up as a U for the opamp but if you change that character to an A and open the model you'll see Annn instead. Every other character, including multiple characters, I changed the value to was mirrored in ltspice when i opend the .asy file.