go/printer: measure lines/construct in generated output rather than incoming source
No change to $GOROOT/src, misc formatting.
Nice side-effect: almost 3% faster runs because it's much faster to compute
line number differences in the generated output than the incoming source.
Benchmark run, best of 5 runs, before and after:
BenchmarkPrint 200
12347587 ns/op
BenchmarkPrint 200
11999061 ns/op
Fixes #4504.
LGTM=adonovan
R=golang-codereviews, adonovan
CC=golang-codereviews
https://golang.org/cl/
69260045