Introduce ref_ptr::new_reference

I noticed a common pattern with gdb::ref_ptr, where callers would
"incref" and then create a new wrapper object, like:

    Py_INCREF (obj);
    gdbpy_ref<> ref (obj);

The ref_ptr constructor intentionally does not acquire a new
reference, but it seemed to me that it would be reasonable to add a
static member function that does so.

In this patch I chose to call the function "new_reference".  I
considered "acquire_reference" as well, but "new" seemed less
ambiguous than "acquire" to me.

ChangeLog
2018-04-30  Tom Tromey  <tom@tromey.com>

	* common/gdb_ref_ptr.h (ref_ptr::new_reference): New static
	method.

gdb/common/gdb_ref_ptr.h

@@ -149,6 +149,13 @@ class ref_ptr
     return m_obj;
   }
 
+  /* Acquire a new reference and return a ref_ptr that owns it.  */
+  static ref_ptr<T, Policy> new_reference (T *obj)
+  {
+    Policy::incref (obj);
+    return ref_ptr<T, Policy> (obj);
+  }
+
  private:
 
   T *m_obj;