Mark namespaces for user defined literals as 'inline'

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@192047 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -4158,10 +4158,7 @@ basic_string<_CharT, _Traits, _Allocator>::__subscriptable(const const_iterator*
 
 #if _LIBCPP_STD_VER > 11 
 // Literal suffixes for basic_string [basic.string.literals]
-// inline // Deviation from N3690.
-//    We believe the inline to be a defect and have submitted an LWG issue.
-//    An LWG issue number has not yet been assigned.
-namespace literals
+inline namespace literals
 {
   inline namespace string_literals
   {