internal/lsp/cmd: print flags when running gopls help

Currently, flags are not being printed when users run `gopls help` because they
never get parsed in Main. This CL fixes that issue.

Updates golang/go#35855

Change-Id: Ic9882d0d2410a0f045aa0ecaa87b36c23eb569fd
Reviewed-on: https://go-review.googlesource.com/c/tools/+/330854
Trust: Jean de Klerk <deklerk@google.com>
Run-TryBot: Jean de Klerk <deklerk@google.com>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Findley <rfindley@google.com>

internal/tool/tool.go

@@ -88,6 +88,7 @@ func Main(ctx context.Context, app Application, args []string) {
 		fmt.Fprintf(s.Output(), "\n\nUsage: %v [flags] %v\n", app.Name(), app.Usage())
 		app.DetailedHelp(s)
 	}
+	addFlags(s, reflect.StructField{}, reflect.ValueOf(app))
 	if err := Run(ctx, app, args); err != nil {
 		fmt.Fprintf(s.Output(), "%s: %v\n", app.Name(), err)
 		if _, printHelp := err.(commandLineError); printHelp {