No functionality change at this time.  I've split _LIBCPP_VISIBLE up into two flags: _LIBCPP_TYPE_VIS and _LIBCPP_FUNC_VIS.  This is in preparation for taking advantage of clang's new __type_visibility__ attribute.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@176593 91177308-0d34-0410-b5e6-96231b3b80d8