compiler: panic arguments are empty interface type

After CL 536643 passing NULL as the expected type permitted an untyped
constant expression to remain untyped. Change to passing the empty
interface type.

The panic and print/println functions are the only builtin functions
that turn an untyped constant expression into a regular function call,
and we already handled print/println specially.

The test case is https://go.dev/cl/603096.

Fixes golang/go#68734

Change-Id: I02f353425b1f4cb8253ab7b29d707d93ede62f72
Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/603215
Reviewed-by: Than McIntosh <thanm@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

go/expressions.cc

@@ -11137,6 +11137,12 @@ Builtin_call_expression::do_determine_type(Gogo* gogo,
       is_print = false;
       break;
 
+    case BUILTIN_PANIC:
+      arg_type =
+	Type::make_empty_interface_type(Linemap::predeclared_location());
+      is_print = false;
+      break;
+
     case BUILTIN_PRINT:
     case BUILTIN_PRINTLN:
       // Do not force a large integer constant to "int".