Add conditional noexcept forwarding from free to member swap functions.

This allows std::is_nothrow_swappable{,_with} to work properly.
This only touches user-facing classes, with the following exceptions:
- map,set: only doc changes (with a drive-by missing semicolon fix),
  detail::tree which they derive from already forwards noexcept for swap.
- flat_set: only doc changes for flat_set and a change for
  detail::flat_tree, which it derives from to get swap.
- static_vector: while this adds the forwarding, member static_vector::swap
  is currently never noexcept, although it could be for noexcept-swappable
  and noexcept-move-constructible elements.
- small_vector: there is no free swap, and member swap is never noexcept,
  although it could be for noexcept-swappable and noexcept-move-
  constructible elements. Swapping small_vectors of exact same type goes
  through std::swap, which seems to get noexcept right, and small_vectors
  of different sizes use swap for boost::container::vector, which fails
  to detect when such swap could be noexcept. This patch doesn't touch
  small_vector.
- scoped_allocator: neither member nor free swap are noexcept, although
  they should be, and all other constructions/assignments/comparisons too,
  as they only forward these operations to underlying allocators, which
  are required to not throw exceptions for them. Only static_vector's
  static_storage_allocator might throw for these and it shouldn't appear
  in scoped_allocator. Unsure of the scope of changes needed or whether it
  is even worth it, this patch doesn't touch scoped_allocator.

include/boost/container/string.hpp

@@ -3399,6 +3399,7 @@ operator>=( const basic_string<CharT,Traits,Allocator>& x, BasicStringView<CharT
 // Swap.
 template <class CharT, class Traits, class Allocator>
 inline void swap(basic_string<CharT,Traits,Allocator>& x, basic_string<CharT,Traits,Allocator>& y)
+    BOOST_NOEXCEPT_IF(BOOST_NOEXCEPT(x.swap(y)))
 {  x.swap(y);  }
 
 #ifndef BOOST_CONTAINER_DOXYGEN_INVOKED