[libcxx] Make sure we can build with -fvisibility=hidden on Linux

Summary:
This commit marks a few functions as hidden and removes them from the ABI list
on Linux such that libc++ can be built with -fvisibility=hidden. The functions
marked as hidden by this patch were exported from the shared object only
because they were implicitly instantiated function templates. It is safe
to stop exporting those symbols from the shared object because nobody could
actually depend on them: implicit instantiations are not taken from shared
objects.

The symbols removed in this commit are basically the same that had been
removed in https://reviews.llvm.org/D53868, but that patch had to be reverted
because it broke the build (because the functions were not marked as hidden
like this patch does).

Reviewers: EricWF, mclow.lists

Subscribers: christof, jkorous, dexonsmith, libcxx-commits

Differential Revision: https://reviews.llvm.org/D54639

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@347395 91177308-0d34-0410-b5e6-96231b3b80d8

include/algorithm

@@ -3611,6 +3611,7 @@ __sort4(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
 // stable, 4-10 compares, 0-9 swaps
 
 template <class _Compare, class _ForwardIterator>
+_LIBCPP_HIDDEN
 unsigned
 __sort5(_ForwardIterator __x1, _ForwardIterator __x2, _ForwardIterator __x3,
             _ForwardIterator __x4, _ForwardIterator __x5, _Compare __c)