Remove the non-standard __cpp_lib_allocator_is_always_equal macro and
add the missing macros for P1032R1.

         PR libstdc++/91480
         * include/bits/allocator.h (__cpp_lib_allocator_is_always_equal):
         Remove non-standard macro.
         * include/bits/stl_iterator.h (__cpp_lib_constexpr_iterator): Define
         to indicate P1032R1 support.
         * include/bits/stl_pair.h (__cpp_lib_constexpr_utility): Likewise.
         * include/std/string_view (__cpp_lib_constexpr_string_view): Likewise.
         * include/std/tuple (__cpp_lib_constexpr_tuple): Likewise.
         * include/std/version (__cpp_lib_allocator_is_always_equal): Remove.
         (__cpp_lib_constexpr_iterator, __cpp_lib_constexpr_string_view)
         (__cpp_lib_constexpr_tuple, __cpp_lib_constexpr_utility): Define.
         * testsuite/20_util/function_objects/constexpr_searcher.cc: Check
         feature test macro.
         * testsuite/20_util/tuple/cons/constexpr_allocator_arg_t.cc: Likewise.
         * testsuite/21_strings/basic_string_view/operations/copy/char/
         constexpr.cc: Likewise.
         * testsuite/24_iterators/insert_iterator/constexpr.cc: Likewise.

Tested powerpc64le-linux, committed to master.

I think with Iain's recent addition of __cpp_lib_coroutine and this,
all our feature test macros are present and correct (except for some
still being defined as int values rather than long).

Thanks to Alisdair Meredith for checking them and bringing the four
missing __cpp_lib_constexpr_xxx ones to my attention.