Build with -fvisibility=hidden

Summary:
This change changes the build to use -fvisibility=hidden

The exports this patch removes are symbols that should have never been exported
by the dylib in the first place, and should all be symbols which the linker
won't de-duplicate across SO boundaries, making them safe to remove.

After this change, we should be able to apply `_LIBCPP_HIDDEN` to the versioning namespace without changing the export lists.

Reviewers: ldionne, mclow.lists

Reviewed By: ldionne

Subscribers: smeenai, mgorny, libcxx-commits

Differential Revision: https://reviews.llvm.org/D53868

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@345664 91177308-0d34-0410-b5e6-96231b3b80d8