Fixes #9915.

Uses intrusive is_convertible in MSVC compilers.
Removes some throw conditions in the documentation produced by the allocator copy constructor (as the standard requires no-throw guarantee for those).

include/boost/container/allocator.hpp

@@ -67,7 +67,7 @@ class allocator<void, Version, AllocationDisableMask>
 
    //!Default constructor
    //!Never throws
-   allocator() 
+   allocator()
    {}
 
    //!Constructor from other allocator.