Use string_file::release in some places

I found a few spots like:

    string_file f;
    std::string x = f.string ();

However, string_file::string returns a 'const std::string &'...  so it
seems to me that this must be copying the string (? I find it hard to
reason about this in C++).

This patch changes these spots to use release() instead, which moves
the string.

Reviewed-by: Keith Seitz <keiths@redhat.com>
Reviewed-by: Lancelot Six <lancelot.six@amd.com>

gdb/breakpoint.c

@@ -7581,7 +7581,7 @@ bp_location::to_string () const
   string_file stb;
   ui_out_redirect_pop redir (current_uiout, &stb);
   print_breakpoint_location (this->owner, this);
-  return stb.string ();
+  return stb.release ();
 }
 
 /* Decrement reference count.  If the reference count reaches 0,

gdb/top.c

@@ -647,12 +647,12 @@ execute_fn_to_string (std::string &res, std::function<void(void)> fn,
   catch (...)
     {
       /* Finally.  */
-      res = std::move (str_file.string ());
+      res = str_file.release ();
       throw;
     }
 
   /* And finally.  */
-  res = std::move (str_file.string ());
+  res = str_file.release ();
 }
 
 /* See gdbcmd.h.  */