gdb: make add_com_alias accept target as a cmd_list_element

The alias creation functions currently accept a name to specify the
target command.  They pass this to add_alias_cmd, which needs to lookup
the target command by name.

Given that:

 - We don't support creating an alias for a command before that command
   exists.
 - We always use add_info_alias just after creating that target command,
   and therefore have access to the target command's cmd_list_element.

... change add_com_alias to accept the target command as a
cmd_list_element (other functions are done in subsequent patches).  This
ensures we don't create the alias before the target command, because you
need to get the cmd_list_element from somewhere when you call the alias
creation function.  And it avoids an unecessary command lookup.  So it
seems better to me in every aspect.

gdb/ChangeLog:

	* command.h (add_com_alias): Accept target as
	cmd_list_element.  Update callers.

Change-Id: I24bed7da57221cc77606034de3023fedac015150

gdb/compile/compile.c

@@ -931,7 +931,7 @@ _initialize_compile ()
 					compile_command, _("\
 Command to compile source code and inject it into the inferior."),
 		  &compile_command_list, 1, &cmdlist);
-  add_com_alias ("expression", "compile", class_obscure, 0);
+  add_com_alias ("expression", compile_cmd_element, class_obscure, 0);
 
   const auto compile_opts = make_compile_options_def_group (nullptr);