bug#52448: key-valid-p vs "C-M-x" and "ESC C-x"

[Stefan Kangas]

AFAIU, we want exactly one valid way to write keybindings with
`key-valid-p'.  Should both of these be `key-valid-p'?

1.  (key-description (kbd "ESC C-x"))
    => "C-M-x"

    (key-valid-p "ESC C-x")
    => t

2.  (key-description (kbd "C-M-x"))
    => "C-M-x"

    (key-valid-p "C-M-x")
    => t

However, there is also this, and the comment at the top of keymap.c:

    (kbd "ESC C-x")
    => "^[^X"

    (kbd "M-C-x")
    => [134217752]