libstdc++: Allow std::swap to find overload for std::exception_ptr

The non-member swap for std::exception_ptr is in a nested namespace and
so can only be found by ADL currently. Add a using-declaration so that
qualified std::swap calls will use the std::exception_ptr::swap member,
instead of the generic std::swap.

There's no new test for this, because the generic std::swap works, it
just does more work than is necessary.

Also tell Doxygen to replace the __exception_ptr namespace with
"__unspecified__" in the generate docs, so the real name is not
documented.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Replace __exception_ptr
	with "__unspecified__".
	* libsupc++/exception_ptr.h: Improve doxygen docs.
	(__exception_ptr::swap): Also declare in namespace std.

libstdc++-v3/doc/doxygen/user.cfg.in

@@ -2408,6 +2408,7 @@ PREDEFINED             = __cplusplus=202002L \
                          _GLIBCXX_HAVE_BUILTIN_LAUNDER \
 			 "_GLIBCXX_DOXYGEN_ONLY(X)=X " \
 			 __allocator_base=std::__new_allocator \
+			 __exception_ptr=__unspecified__ \
 
 # If the MACRO_EXPANSION and EXPAND_ONLY_PREDEF tags are set to YES then this
 # tag can be used to specify a list of macro names that should be expanded. The

libstdc++-v3/libsupc++/exception_ptr.h

@@ -65,9 +65,12 @@ namespace std _GLIBCXX_VISIBILITY(default)
 
   using __exception_ptr::exception_ptr;
 
-  /** Obtain an exception_ptr to the currently handled exception. If there
-   *  is none, or the currently handled exception is foreign, return the null
-   *  value.
+  /** Obtain an exception_ptr to the currently handled exception.
+   *
+   * If there is none, or the currently handled exception is foreign,
+   * return the null value.
+   *
+   * @since C++11
    */
   exception_ptr current_exception() _GLIBCXX_USE_NOEXCEPT;
 
@@ -79,10 +82,16 @@ namespace std _GLIBCXX_VISIBILITY(default)
 
   namespace __exception_ptr
   {
-    using std::rethrow_exception;
+    using std::rethrow_exception; // So that ADL finds it.
 
     /**
      *  @brief An opaque pointer to an arbitrary exception.
+     *
+     * The actual name of this type is unspecified, so the alias
+     * `std::exception_ptr` should be used to refer to it.
+     *
+     *  @headerfile exception
+     *  @since C++11 (but usable in C++98 as a GCC extension)
      *  @ingroup exceptions
      */
     class exception_ptr
@@ -231,6 +240,8 @@ namespace std _GLIBCXX_VISIBILITY(default)
 
   } // namespace __exception_ptr
 
+  using __exception_ptr::swap; // So that std::swap(exp1, exp2) finds it.
+
   /// Obtain an exception_ptr pointing to a copy of the supplied object.
 #if (__cplusplus >= 201103L && __cpp_rtti) || __cpp_exceptions
   template<typename _Ex>