expand support for noreturn declarations

Although gcc specifies support for declaring a function as
noreturn after the function name and parameters, other compilers
do so via an attribute at the start of the declaration. Add some
macro support for the attribute-at-the-beginning method:
  o MS Visual Studio compiler
  o Upcoming C23 standard (untested at this point)

win/tty/wintty.c

@@ -1537,6 +1537,8 @@ tty_exit_nhwindows(const char *str)
     iflags.window_inited = 0;
 }
 
+DISABLE_WARNING_UNREACHABLE_CODE
+
 winid
 tty_create_nhwindow(int type)
 {
@@ -1651,6 +1653,8 @@ tty_create_nhwindow(int type)
     return newid;
 }
 
+RESTORE_WARNING_UNREACHABLE_CODE
+
 static void
 erase_menu_or_text(winid window, struct WinDesc *cw, boolean clear)
 {