bug#49689: [FEATURE REQUEST] Explicitly report change of input method

[Juri Linkov]

> FWIW, sometimes I wish Emacs was a little more explicit about changing
> input methods, even more so now that transient input methods are a
> thing.  Sometimes, I find myself in an odd state of using an input
> method when I activate "compose" transiently (I have it bound to M-m):
> this could just be my buggy command though.  The message "Loading ..."
> helps but not when you switch the second time.  The mode-line indicator
> is not a great solution when the indicator is a single letter (I use a
> proportional font for the mode-line).

Then you might want to add an advice to the function that toggles
the input method and display a message about the currently active
input method.