Allow std::vector to move construct its allocator

[fidget324]

Summary

Fix an issue that was preventing std::vector from invoking the move
constructor on its allocator when appropriate.

Added a constructor to __vector_base which accepts an rvalue reference
to the allocator, thus allowing the move constructor to be invoked.
Previously, only a const lvalue reference was being accepted.

This fixes bug 37694: https://bugs.llvm.org/show_bug.cgi?id=37694.

[EricWF]

@fidget324 Thanks for the patch! Unfortunatly I didn't know you were planning on contributing a fix, so I went ahead and fixed it myself in r334053.

[fidget324]

That's fine. Your patch was much more thorough, anyways.

include/vector

protected:
     _LIBCPP_INLINE_VISIBILITY
     const pointer& __end_cap() const _NOEXCEPT
         {return __end_cap_.first();}
 
     _LIBCPP_INLINE_VISIBILITY
     __vector_base()
         _NOEXCEPT_(is_nothrow_default_constructible<allocator_type>::value);
     _LIBCPP_INLINE_VISIBILITY __vector_base(const allocator_type& __a);
+    _LIBCPP_INLINE_VISIBILITY __vector_base(allocator_type&& __a);
     ~__vector_base();
 
     _LIBCPP_INLINE_VISIBILITY
     void clear() _NOEXCEPT {__destruct_at_end(__begin_);}
     _LIBCPP_INLINE_VISIBILITY
     size_type capacity() const _NOEXCEPT
         {return static_cast<size_type>(__end_cap() - __begin_);}
 

 __vector_base<_Tp, _Allocator>::__vector_base(const allocator_type& __a)
     : __begin_(nullptr),
       __end_(nullptr),
       __end_cap_(nullptr, __a)
 {
 }
 
 template <class _Tp, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
+__vector_base<_Tp, _Allocator>::__vector_base(allocator_type&& __a)
+    : __begin_(nullptr),
+      __end_(nullptr),
+      __end_cap_(nullptr, std::move(__a))
+{
+}
+
+template <class _Tp, class _Allocator>
 __vector_base<_Tp, _Allocator>::~__vector_base()
 {
     if (__begin_ != nullptr)
     {
         clear();
         __alloc_traits::deallocate(__alloc(), __begin_, capacity());
     }
 }