go/parser: correct position of empty statement ';'

(caused certain files to not be idempotent under gofmt)

- corrected golden files for go/printer
- slightly simplified some code in nodes.go (no impact on formatting)
- these changes have no impact on gofmt output of .go files under src, misc

fallthrough statement considered harmful!

R=rsc
CC=golang-dev
https://golang.org/cl/1593042

src/pkg/go/parser/parser.go

@@ -1716,8 +1716,7 @@ func (p *parser) parseForStmt() ast.Stmt {
 		var key, value ast.Expr
 		switch len(as.Lhs) {
 		case 2:
-			value = as.Lhs[1]
-			fallthrough
+			key, value = as.Lhs[0], as.Lhs[1]
 		case 1:
 			key = as.Lhs[0]
 		default:
@@ -1785,8 +1784,8 @@ func (p *parser) parseStmt() (s ast.Stmt) {
 	case token.FOR:
 		s = p.parseForStmt()
 	case token.SEMICOLON:
+		s = &ast.EmptyStmt{p.pos}
 		p.next()
-		fallthrough
 	case token.RBRACE:
 		// a semicolon may be omitted before a closing "}"
 		s = &ast.EmptyStmt{p.pos}

src/pkg/go/printer/nodes.go

@@ -1030,12 +1030,12 @@ func (p *printer) stmt(stmt ast.Stmt, nextIsRBrace bool, multiLine *bool) {
 		// a "correcting" unindent immediately following a line break
 		// is applied before the line break if there is no comment
 		// between (see writeWhitespace)
-		p.print(unindent, s.Pos())
+		p.print(unindent)
 		p.expr(s.Label, multiLine)
 		p.print(token.COLON, indent)
-		if _, isEmpty := s.Stmt.(*ast.EmptyStmt); isEmpty {
+		if e, isEmpty := s.Stmt.(*ast.EmptyStmt); isEmpty {
 			if !nextIsRBrace {
-				p.print(newline, s.Stmt.Pos(), token.SEMICOLON)
+				p.print(newline, e.Pos(), token.SEMICOLON)
 				break
 			}
 		} else {

src/pkg/go/printer/testdata/statements.golden

@@ -259,11 +259,11 @@ L:	// no semicolon needed
 func _() {
 	switch 0 {
 	case 0:
-	L0:	// semicolon required
-		;
+	L0:
+		;	// semicolon required
 	case 1:
-	L1:	// semicolon required
-		;
+	L1:
+		;	// semicolon required
 	default:
 	L2:	// no semicolon needed
 	}