Fix const correctness bug in __move_assign.  Found and fixed by Ion Gaztañaga.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@139032 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/include/string b/include/string
index a1dba31..2041510 100644
--- a/include/string
+++ b/include/string
@@ -1593,7 +1593,7 @@
 
     _LIBCPP_INLINE_VISIBILITY
     void
-    __move_assign_alloc(const basic_string& __str)
+    __move_assign_alloc(basic_string& __str)
         _NOEXCEPT_(
             !__alloc_traits::propagate_on_container_move_assignment::value ||
             is_nothrow_move_assignable<allocator_type>::value)
@@ -1601,14 +1601,14 @@
                       __alloc_traits::propagate_on_container_move_assignment::value>());}
 
     _LIBCPP_INLINE_VISIBILITY
-    void __move_assign_alloc(const basic_string& __c, true_type)
+    void __move_assign_alloc(basic_string& __c, true_type)
         _NOEXCEPT_(is_nothrow_move_assignable<allocator_type>::value)
         {
             __alloc() = _VSTD::move(__c.__alloc());
         }
 
     _LIBCPP_INLINE_VISIBILITY
-    void __move_assign_alloc(const basic_string& __c, false_type)
+    void __move_assign_alloc(basic_string& __c, false_type)
         _NOEXCEPT
         {}