[libcxx] Mark stray symbols as hidden to try and fix the build

r347395 changed the ABI list on Linux, but two of those symbols are still
being exported from the shared object:

    _ZSt18make_exception_ptrINSt3__112future_errorEESt13exception_ptrT_
    _ZNSt3__1plIcNS_11char_traitsIcEENS_9allocatorIcEEEENS_12basic_stringIT_T0_T1_EERKS9_PKS6_

This commit makes sure those symbols are not exported, as they should be.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347399 91177308-0d34-0410-b5e6-96231b3b80d8

include/exception

@@ -164,7 +164,7 @@ public:
 };
 
 template<class _Ep>
-exception_ptr
+_LIBCPP_INLINE_VISIBILITY exception_ptr
 make_exception_ptr(_Ep __e) _NOEXCEPT
 {
 #ifndef _LIBCPP_NO_EXCEPTIONS
@@ -223,7 +223,7 @@ _LIBCPP_NORETURN _LIBCPP_FUNC_VIS void rethrow_exception(exception_ptr p);
 template <class _E> void *__GetExceptionInfo(_E);
 
 template<class _Ep>
-exception_ptr
+_LIBCPP_INLINE_VISIBILITY exception_ptr
 make_exception_ptr(_Ep __e) _NOEXCEPT
 {
   return __copy_exception_ptr(_VSTD::addressof(__e), __GetExceptionInfo(__e));

include/string

@@ -4036,7 +4036,7 @@ operator+(_CharT __lhs, const basic_string<_CharT,_Traits,_Allocator>& __rhs)
 }
 
 template<class _CharT, class _Traits, class _Allocator>
-_LIBCPP_INLINE_VISIBILITY
+inline _LIBCPP_INLINE_VISIBILITY
 basic_string<_CharT, _Traits, _Allocator>
 operator+(const basic_string<_CharT, _Traits, _Allocator>& __lhs, const _CharT* __rhs)
 {