debug/dwarf, encoding/asn1, go/ast: fix old comments

The debug/dwarf and encoding/asn1 examples were added in 2009, a few
months before Go added implicit semicolons, and never updated.

The go/ast node types have always been named just "Expr", "Stmt", and
"Decl", so the comments about "ExprNode", "StmtNode", and "DeclNode"
were likely just mistaken because the interface tag methods are
"exprNode", "stmtNode", and "declNode", respectively.

Change-Id: I9d138cc3a16c1a51453da1406914d7b320bf6270
Reviewed-on: https://go-review.googlesource.com/7980Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/debug/dwarf/entry.go

@@ -112,7 +112,7 @@ type Field struct {
 //
 // A common idiom is to merge the check for nil return with
 // the check that the value has the expected dynamic type, as in:
-//	v, ok := e.Val(AttrSibling).(int64);
+//	v, ok := e.Val(AttrSibling).(int64)
 //
 func (e *Entry) Val(a Attr) interface{} {
 	for _, f := range e.Field {

src/encoding/asn1/marshal.go

@@ -18,7 +18,7 @@ import (
 // A forkableWriter is an in-memory buffer that can be
 // 'forked' to create new forkableWriters that bracket the
 // original.  After
-//    pre, post := w.fork();
+//    pre, post := w.fork()
 // the overall sequence of bytes represented is logically w+pre+post.
 type forkableWriter struct {
 	*bytes.Buffer

src/go/ast/ast.go

@@ -486,7 +486,7 @@ func (x *MapType) End() token.Pos       { return x.Value.End() }
 func (x *ChanType) End() token.Pos      { return x.Value.End() }
 
 // exprNode() ensures that only expression/type nodes can be
-// assigned to an ExprNode.
+// assigned to an Expr.
 //
 func (*BadExpr) exprNode()        {}
 func (*Ident) exprNode()          {}
@@ -787,7 +787,7 @@ func (s *ForStmt) End() token.Pos    { return s.Body.End() }
 func (s *RangeStmt) End() token.Pos  { return s.Body.End() }
 
 // stmtNode() ensures that only statement nodes can be
-// assigned to a StmtNode.
+// assigned to a Stmt.
 //
 func (*BadStmt) stmtNode()        {}
 func (*DeclStmt) stmtNode()       {}
@@ -951,7 +951,7 @@ func (d *FuncDecl) End() token.Pos {
 }
 
 // declNode() ensures that only declaration nodes can be
-// assigned to a DeclNode.
+// assigned to a Decl.
 //
 func (*BadDecl) declNode()  {}
 func (*GenDecl) declNode()  {}