gotype: provide -comments flag

When debugging ASTs, it's useful to also
see the comments on occasion. Usage:

gotype -ast -comments file.go

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5703043

src/pkg/exp/gotype/doc.go

@@ -34,6 +34,8 @@ The flags are:
 		Verbose mode.
 
 Debugging flags:
+	-comments
+		Parse comments (ignored if -ast not set).
 	-ast
 		Print AST (disables concurrent parsing).
 	-trace

src/pkg/exp/gotype/gotype.go

@@ -27,6 +27,7 @@ var (
 	allErrors = flag.Bool("e", false, "print all (including spurious) errors")
 
 	// debugging support
+	parseComments = flag.Bool("comments", false, "parse comments (ignored if -ast not set)")
 	printTrace    = flag.Bool("trace", false, "print parse trace")
 	printAST      = flag.Bool("ast", false, "print AST")
 )
@@ -73,6 +74,9 @@ func parse(fset *token.FileSet, filename string, src []byte) *ast.File {
 	if *allErrors {
 		mode |= parser.SpuriousErrors
 	}
+	if *parseComments && *printAST {
+		mode |= parser.ParseComments
+	}
 	if *printTrace {
 		mode |= parser.Trace
 	}