cmd/internal/src: do we need to carry around absFilename (and symFilename) ?

[griesemer]

pos.go defines a PosBase which carries an andFilename. absFilenames are produced from the regular filename when the PosBase is created and then don't change. It seems that we could just as well produce the absFilename only at the end, when we need it (to flow into the compiler's generated binary). Investigate.

Reminder issue.

(See also #22660.)

[griesemer]

Not urgent.