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/static_vector.hpp

@@ -1169,6 +1169,7 @@ public:
 #else
 
    BOOST_CONTAINER_FORCEINLINE friend void swap(static_vector &x, static_vector &y)
+       BOOST_NOEXCEPT_IF(BOOST_NOEXCEPT(x.swap(y)))
    {
       x.swap(y);
    }
@@ -1275,13 +1276,15 @@ bool operator>= (static_vector<V, C1, O1> const& x, static_vector<V, C2, O2> con
 //! @par Complexity
 //!   Linear O(N).
 template<typename V, std::size_t C1, std::size_t C2, class O1, class O2>
-inline void swap(static_vector<V, C1, O1> & x, static_vector<V, C2, O2> & y);
+inline void swap(static_vector<V, C1, O1> & x, static_vector<V, C2, O2> & y)
+    BOOST_NOEXCEPT_IF(BOOST_NOEXCEPT(x.swap(y)));
 
 #else
 
 template<typename V, std::size_t C1, std::size_t C2, class O1, class O2>
 inline void swap(static_vector<V, C1, O1> & x, static_vector<V, C2, O2> & y
       , typename dtl::enable_if_c< C1 != C2>::type * = 0)
+    BOOST_NOEXCEPT_IF(BOOST_NOEXCEPT(x.swap(y)))
 {
    x.swap(y);
 }