Temporary restriction that extend base is first in a class to avoid #2994 (#2995)

Closes #2994

---------

Co-authored-by: Geoff Romer <gromer@google.com>

explorer/testdata/class/fail_extend_after_var.carbon

@@ -11,7 +11,7 @@ base class C {
 
 class E {
   var x: i32;
-  // CHECK:STDERR: COMPILATION ERROR: fail_extend_after_var.carbon:[[@LINE+1]]: `extend base:` declaration must not be after `var` or `mix` declarations in a class.
+  // CHECK:STDERR: COMPILATION ERROR: fail_extend_after_var.carbon:[[@LINE+1]]: `extend base:` declarations after the first declaration in the class are not yet supported
   extend base: C;
 }
 

explorer/testdata/class/fail_extend_member_class.carbon

@@ -0,0 +1,22 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+// AUTOUPDATE
+
+package ExplorerTest api;
+
+class E {
+  base class C {
+  }
+
+  // This should be allowed, but Explorer forbids it until the problem from
+  // https://github.com/carbon-language/carbon-lang/issues/2994 can be fixed.
+
+  // CHECK:STDERR: COMPILATION ERROR: fail_extend_member_class.carbon:[[@LINE+1]]: `extend base:` declarations after the first declaration in the class are not yet supported
+  extend base: C;
+}
+
+fn Main() -> i32 {
+  return 0;
+}