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/regexp/syntax/parse.go b/src/regexp/syntax/parse.go
index 8c6d43a706..8b5b32a786 100644
--- a/src/regexp/syntax/parse.go
+++ b/src/regexp/syntax/parse.go
@@ -1626,7 +1626,6 @@ func (p *parser) parseClass(s string) (rest string, err error) {
 // cleanClass sorts the ranges (pairs of elements of r),
 // merges them, and eliminates duplicates.
 func cleanClass(rp *[]rune) []rune {
-
 	// Sort by lo increasing, hi decreasing to break ties.
 	sort.Sort(ranges{rp})