go/types: add -panic flag to gotype command for debugging

Setting -panic will cause gotype to panic with the first reported
error, producing a stack trace for debugging.

For #23914.

Change-Id: I40c41cf10aa13d1dd9a099f727ef4201802de13a
Reviewed-on: https://go-review.googlesource.com/96375Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

src/go/types/gotype.go

@@ -53,6 +53,8 @@ Flags controlling additional output:
 		print parse trace (forces -seq)
 	-comments
 		parse comments (ignored unless -ast or -trace is provided)
+	-panic
+		panic on first error
 
 Examples:
 
@@ -105,6 +107,7 @@ var (
 	printAST      = flag.Bool("ast", false, "print AST (forces -seq)")
 	printTrace    = flag.Bool("trace", false, "print parse trace (forces -seq)")
 	parseComments = flag.Bool("comments", false, "parse comments (ignored unless -ast or -trace is provided)")
+	panicOnError  = flag.Bool("panic", false, "panic on first error")
 )
 
 var (
@@ -164,6 +167,9 @@ func usage() {
 }
 
 func report(err error) {
+	if *panicOnError {
+		panic(err)
+	}
 	scanner.PrintError(os.Stderr, err)
 	if list, ok := err.(scanner.ErrorList); ok {
 		errorCount += len(list)