runtime: In Frames.Next, delay file/line lookup until just before return

runtime: In Frames.Next, delay file/line lookup until just before return

That way we will never have to look up the file/line for the frame
that's next to be returned when the user stops calling Next.

For the benchmark from #32093:

name      old time/op  new time/op  delta
Helper-4   948ns ± 1%   836ns ± 3%  -11.89%  (p=0.000 n=9+9)

(#32093 was fixed with a more specific, and better, fix, but this
fix is much more general.)

Change-Id: I89e796f80c9706706d8d8b30eb14be3a8a442846
Reviewed-on: https://go-review.googlesource.com/c/go/+/178077
Run-TryBot: Keith Randall <khr@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>