libstdc++: Also disable caching of reverse_view::begin() for common_ranges

When the underlying range models common_range, then reverse_view::begin() is
already O(1) without caching.  So we should disable the cache in this case too.

libstdc++-v3/ChangeLog:

	* include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
	whenever the underlying range models common_range.

libstdc++-v3/ChangeLog

 2020-02-28  Patrick Palka  <ppalka@redhat.com>
 
+	* include/std/ranges (reverse_view::_S_needs_cached_begin): Set to false
+	whenever the underlying range models common_range.
+
 	* include/std/ranges (__detail::_CachedPosition): New struct.
 	(views::filter_view::_S_needs_cached_begin): New member variable.
 	(views::filter_view::_M_cached_begin): New member variable.

libstdc++-v3/include/std/ranges

@@ -3177,7 +3177,8 @@ namespace views
     private:
       _Vp _M_base = _Vp();
 
-      static constexpr bool _S_needs_cached_begin = !random_access_range<_Vp>;
+      static constexpr bool _S_needs_cached_begin
+	= !common_range<_Vp> && !random_access_range<_Vp>;
       [[no_unique_address]]
 	__detail::__maybe_empty_t<_S_needs_cached_begin,
 				  __detail::_CachedPosition<_Vp>> _M_cached_begin;