go/printer: forbid empty line before first comment in block

To improve readability when exported fields are removed,
forbid the printer from emitting an empty line before the first comment
in a const, var, or type block.
Also, when printing the "Has filtered or unexported fields." message,
add an empty line before it to separate the message from the struct
or interfact contents.

Before the change:
<<<
type NamedArg struct {

        // Name is the name of the parameter placeholder.
        //
        // If empty, the ordinal position in the argument list will be
        // used.
        //
        // Name must omit any symbol prefix.
        Name string

        // Value is the value of the parameter.
        // It may be assigned the same value types as the query
        // arguments.
        Value interface{}
        // contains filtered or unexported fields
}
>>>

After the change:
<<<
type NamedArg struct {
        // Name is the name of the parameter placeholder.
        //
        // If empty, the ordinal position in the argument list will be
        // used.
        //
        // Name must omit any symbol prefix.
        Name string

        // Value is the value of the parameter.
        // It may be assigned the same value types as the query
        // arguments.
        Value interface{}

        // contains filtered or unexported fields
}
>>>

Fixes #18264

Change-Id: I9fe17ca39cf92fcdfea55064bd2eaa784ce48c88
Reviewed-on: https://go-review.googlesource.com/71990
Run-TryBot: Joe Tsai <thebrokentoaster@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

1 files changed, 0 insertions, 1 deletions

diff --git a/src/net/http/requestwrite_test.go b/src/net/http/requestwrite_test.go
index eb65b9f736..74b01db6da 100644
--- a/src/net/http/requestwrite_test.go
+++ b/src/net/http/requestwrite_test.go
@@ -813,7 +813,6 @@ func TestRequestWriteError(t *testing.T) {
 // try to restore it. It always dumps the whole body.
 // And it doesn't support https.
 func dumpRequestOut(req *Request, onReadHeaders func()) ([]byte, error) {
-
 	// Use the actual Transport code to record what we would send
 	// on the wire, but not using TCP.  Use a Transport with a
 	// custom dialer that returns a fake net.Conn that waits