Allow suppressing the "lexical arg shadows dynbound var" warning

Allow suppressing the "lexical arg shadows dynbound var" warning

In most cases the right way to fix this warning is by renaming
the offending argument, but in some cases this is inconvenient, as is
the case in `cl-defstruct` where arg names are imposed by slot names.

This patch also happens to fix a few bugs along the way:
- miscompilation of (lambda (gcs-done) (lambda (x) (+ x gcs-done)))
- errors about void function `byte-compile-warn-x` if the warning was
  emitted via `cconv-fv` when bytecomp was not loaded.
Oh, and it improves the warning by making the location info slightly
more precise.

* lisp/emacs-lisp/cconv.el (cconv--analyze-function): Remove this warning.
* lisp/emacs-lisp/bytecomp.el (byte-compile-check-lambda-list):
Warn about it here instead.  Let `with-suppressed-warnings` control it
under `lexical`.