Consolidate existing warnings about unused return values

Move the warning about unused return values from calls to
side-effect-free functions from the source-level optimiser to the code
generator, where it can be unified with the special-purpose warning
about unused values from `mapcar`.  This change also cures spurious
duplicate warnings about the same code, makes the warnings amenable to
suppression through `with-suppressed-warnings`, and now warns about
some unused values that weren't caught before.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-form-code-walker):
Move warning away from here.
* lisp/emacs-lisp/byte-run.el (with-suppressed-warnings):
* lisp/emacs-lisp/bytecomp.el (byte-compile-warnings):
Doc string updates.
(byte-compile-form): Put the new warnings here.
(byte-compile-normal-call): Move mapcar warning away from here.
* lisp/emacs-lisp/bytecomp.el (byte-compile-ignore):
Compile args to `ignore` for value to avoid unused-value warnings, and
then discard the generated values immediately thereafter.  Mostly this
does not affect the generated code but in rare cases it might result
in slightly worse code.
* test/lisp/emacs-lisp/bytecomp-tests.el
(bytecomp-test--with-suppressed-warnings): Adapt test.

lisp/emacs-lisp/byte-opt.el

@@ -506,13 +506,7 @@ for speeding up processing.")
       ((guard (when for-effect
 		(if-let ((tmp (byte-opt--fget fn 'side-effect-free)))
 		    (or byte-compile-delete-errors
-		        (eq tmp 'error-free)
-		        (progn
-			  (byte-compile-warn-x
-                           form
-                           "value returned from %s is unused"
-			   form)
-			  nil)))))
+		        (eq tmp 'error-free)))))
        (byte-compile-log "  %s called for effect; deleted" fn)
        (byte-optimize-form (cons 'progn (cdr form)) t))