Add -Wshadowed-toplevel.

* module/language/tree-il/analyze.scm (shadowed-toplevel-analysis): New
variable.
* module/language/tree-il/compile-cps.scm (%warning-passes): Add it.
* module/system/base/message.scm (%warning-types): Add it.
* test-suite/tests/tree-il.test ("warnings")["shadowed-toplevel"]: New
test prefix.
* module/ice-9/boot-9.scm (%auto-compilation-options): Add it.
* doc/ref/api-evaluation.texi (Compilation): Add 'shadowed-toplevel' and
'macro-use-before-definition'.

module/system/base/message.scm

@@ -1,6 +1,6 @@
 ;;; User interface messages
 
-;; Copyright (C) 2009, 2010, 2011, 2012 Free Software Foundation, Inc.
+;; Copyright (C) 2009, 2010, 2011, 2012, 2018 Free Software Foundation, Inc.
 
 ;;; This library is free software; you can redistribute it and/or
 ;;; modify it under the terms of the GNU Lesser General Public
@@ -109,6 +109,13 @@
                (emit port "~A: warning: possibly unused local top-level variable `~A'~%"
                      loc name)))
 
+           (shadowed-toplevel
+            "report shadowed top-level variables"
+            ,(lambda (port loc name previous-loc)
+               (emit port "~A: warning: shadows previous definition of `~A' at ~A~%"
+                     loc name
+                     (location-string previous-loc))))
+
            (unbound-variable
             "report possibly unbound variables"
             ,(lambda (port loc name)