[libcxx] Make sure operator+ is declared with the right visibility attribute

Otherwise, Clang complains about internal_linkage not being applied to the
first declaration of the operator (and rightfully so).

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347400 91177308-0d34-0410-b5e6-96231b3b80d8

include/string

@@ -580,6 +580,7 @@ basic_string<_CharT, _Traits, _Allocator>
 operator+(_CharT __x, const basic_string<_CharT,_Traits,_Allocator>& __y);
 
 template<class _CharT, class _Traits, class _Allocator>
+inline _LIBCPP_INLINE_VISIBILITY
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __x, const _CharT* __y);