bug#33901: 26.1; cl-letf is unexpectedly not autoloaded

[Markus Triska]

When I invoke Emacs via:

    $ emacs -Q --eval '(cl-letf ((x t)) x)'

then it displays:

    eval: Symbol’s function definition is void: cl-letf

However, the documentation of cl-letf states:

    "cl-letf is an autoloaded Lisp macro ... "

Hence, I expect cl-letf to be autoloaded in that case. Could this be
changed to work? Thank you!

In GNU Emacs 26.1 (build 1, x86_64-apple-darwin15.3.0, X toolkit, Xaw scroll 
bars)
 of 2018-09-22 built on mt-mbpro
Windowing system distributor 'The X.Org Foundation', version 11.0.11502000