libstdc++: Fix regression in memory use when constructing paths

The changes in r12-4381 were intended to reduce memory usage, but
replacing the __contiguous constant in __string_from_range with the new
__is_contiguous variable template caused a regression. The old code
checked is_pointer_v<decltype(std::__niter_base(__first))> but he new
code just checks is_pointer_v<_InputIterator>. This means that we no
longer recognise basic_string::iterator and vector::iterator as
contiguous, and so return a temporary basic_string instead of a
basic_string_view. This only affects C++17 mode, because the
std::contiguous_iterator concept is used in C++20 which gives the right
answer for __normal_iterator (and more types as well).

The fix is to specialize the new __is_contiguous variable template so it
is true for __normal_iterator<T*, C> specializations. The new partial
specializations are defined for C++20 too, because it should be cheaper
to match the partial specialization than to check whether the
std::contiguous_iterator concept is satisfied.

libstdc++-v3/ChangeLog:

	* include/bits/fs_path.h (__detail::__is_contiguous): Add
	partial specializations for pointers and __normal_iterator.

libstdc++-v3/include/bits/fs_path.h

@@ -158,9 +158,16 @@ namespace __detail
     constexpr bool __is_contiguous = std::contiguous_iterator<_Iter>;
 #else
   template<typename _Iter>
-    constexpr bool __is_contiguous = is_pointer_v<_Iter>;
+    constexpr bool __is_contiguous = false;
 #endif
 
+  template<typename _Tp>
+    constexpr bool __is_contiguous<_Tp*> = true;
+
+  template<typename _Tp, typename _Seq>
+    constexpr bool
+    __is_contiguous<__gnu_cxx::__normal_iterator<_Tp*, _Seq>> = true;
+
 #if !defined _GLIBCXX_FILESYSTEM_IS_WINDOWS && defined _GLIBCXX_USE_CHAR8_T
   // For POSIX treat char8_t sequences as char without encoding conversions.
   template<typename _EcharT>