bug#24765: 26.0.50; Incorrect Edebug spec for cl-letf, cl-letf*

[Gemini Lasswell]

The Edebug spec for cl-letf and cl-letf* doesn't handle the place as
plain variable exception mentioned in the documentation of cl-letf.

To reproduce (emacs -Q or otherwise), enter this code into *scratch*:

(require 'cl-lib)
(defun my-letf-bug (things)
  (cl-letf* ((new-thing 'a)
            ((car things) new-thing))
    (message "%s" things)))

(my-letf-bug (list 1 2 3))

If you evaluate this without the debugger it will give the expected
result, (a 2 3). But try:

M-x edebug-all-defs RET
M-x eval-buffer RET
g

Result: Symbol's value as variable is void: new-thing



In GNU Emacs 26.0.50.4 (x86_64-apple-darwin15.6.0, NS appkit-1404.47 Version 
10.11.6 (Build 15G1004))
 of 2016-10-20 built on rainbow.local
Repository revision: 9e1e257ea8605354d15b4bd93e0af6188aae40db
Windowing system distributor 'Apple', version 10.3.1404
Recent messages:
Symbol’s value as variable is void: new-thing
funcall-interactively: End of buffer [4 times]
Entering debugger...
Back to top level
nil
Mark set
Edebug: my-letf-bug
Go...
Symbol's value as variable is void: new-thing
Go...
edebug-signal: Symbol’s value as variable is void: new-thing
Configured using:
 'configure --with-ns --disable-ns-self-contained'

Configured features:
JPEG RSVG IMAGEMAGICK NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS
NS

Important settings:
  value of $LANG: en_US.UTF-8
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Major mode: Emacs-Lisp

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Load-path shadows:
None found.

Features:
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Memory information:
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