Currently, it separates comments from rest of the AST. This causes problems when
long counter increment statements are added before compiler directives.
See Issue #17315.

This change moves comments handling into AST Visitor so that when printer prints
code from AST, position of compiler directives relative to the associated function
is preserved.

Tested with https://gist.github.com/dhananjay92/837df6bc1f171b1350f85d7a7d59ca1e
and unit test.

Fixes #17315

Change-Id: I61a80332fc1923de6fc59ff63b953671598071fa
Reviewed-on: https://go-review.googlesource.com/30161Reviewed-by: Rob Pike <r@golang.org>