Rollback clang's _NonNull attribute, too many warnings in older compilers. Replaced with BOOST_ASSERT null checks and added some corner cases to the test-suite.

Author: igaztanaga

doc/container.qbk

@@ -1462,7 +1462,8 @@ use [*Boost.Container]? There are several reasons for that:
 * Updated `basic_string`:
    * Added missing `string_view` members and updated `operator[]` to be able to return the terminating null.
    * Added C++20 `starts_with`/`ends_with` and C++23 `contains` overloads.
-   * Added Clang `nullability` checks for `basic_string` methods.
+   * Added null pointer checks for `basic_string` methods taking `const CharT*` parameters.
+   * Deleted constructor from `std::nullptr_t`.
 * Fixed bugs/issues:
    * [@https://github.com/boostorg/container/issues/323 GitHub #323: ['"flat_tree::try_emplace UB"]].
    * [@https://github.com/boostorg/container/issues/328 GitHub #328: ['"boost::container::deque stores a redundant copy of the allocator, increasing size"]].

include/boost/container/detail/workaround.hpp

@@ -252,18 +252,4 @@ namespace boost {
     #define BOOST_CONTAINER_NOVTABLE
 #endif
 
-#if defined(__has_feature)
-#  if __has_feature(nullability)
-#     define BOOST_CONTAINER_NONNULL_SUPPORTED
-#     define BOOST_CONTAINER_NONNULL  _Nonnull
-#     define BOOST_CONTAINER_NULLABLE _Nullable
-#  endif
-#endif
-
-#if !defined(BOOST_CONTAINER_NONNULL_SUPPORTED)
-#  define BOOST_CONTAINER_NONNULL
-#  define BOOST_CONTAINER_NULLABLE
-#  undef BOOST_CONTAINER_NONNULL_SUPPORTED
-#endif
-
 #endif   //#ifndef BOOST_CONTAINER_DETAIL_WORKAROUND_HPP