Don't propagate lexical variables into inlined functions

Don't propagate lexical variables into inlined functions

Functions compiled when inlined (thus from inside the optimiser)
mustn't retain the lexical environment of the caller or there will be
tears.  See discussion at
https://lists.gnu.org/archive/html/emacs-devel/2021-05/msg01227.html .

Bug found by Stefan Monnier.

* lisp/emacs-lisp/byte-opt.el (byte-compile-inline-expand):
Bind byte-optimize--lexvars to nil when re-entering the compiler
recursively.
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-alpha.el:
* test/lisp/emacs-lisp/bytecomp-resources/bc-test-beta.el: New files.
* test/lisp/emacs-lisp/bytecomp-tests.el (bytecomp-defsubst): New test.