Golang cmd/compile/internal/syntax: StartPos incorrect for KeyValueExpr arguments

Reminder issue. See https://go-review.googlesource.com/c/go/+/591256/1/src/cmd/compile/internal/syntax/positions.go .

Trivial fix. Consider for 1.23 to get it out of the way but no problem if it doesn't make it.

Comment From: gopherbot

Change https://go.dev/cl/591695 mentions this issue: cmd/compile/internal/syntax: return correct start pos for KeyValueExprs