Fix local defvar scoping error (bug#46387)

Fix local defvar scoping error (bug#46387)

This bug was introduced by the lexical variable constant propagation
mechanism.  It was discovered by Michael Heerdegen.

* lisp/emacs-lisp/byte-opt.el (byte-optimize-let-form)
(byte-optimize-body): Let the effects of a local defvar declaration be
scoped by let and let*, not any arbitrary Lisp expression body (such
as progn).
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-tests--get-vars)
(bytecomp-local-defvar): New test.