c++: beware of -Wshadow

This line:

    slice<stack_symbol_type, stack_type> slice (yystack_, yylen);

triggers warnings:

    parse.h:1790:11: note: shadowed declaration is here

Reported by Frank Heckenbach.
http://lists.gnu.org/archive/html/bug-bison/2019-01/msg00002.html

* configure.ac (warn_c): Move -Wshadow to...
(warn_common): here.
* data/skeletons/stack.hh (slice): Define as an inner class of stack.
* data/skeletons/lalr1.cc: Adjust.
Rename the variable as 'range' instead of 'slice'.

tests/c++.at

@@ -420,8 +420,7 @@ namespace yy
 {
   static
   ]AT_YYLEX_PROTOTYPE[
-  {]AT_LOCATION_IF([
-    typedef parser::location_type location;])[
+  {
     // The 5 is a syntax error whose recovery requires that we discard
     // the lookahead.  This tests a regression, see
     // <http://savannah.gnu.org/support/?108481>.