[libc++] Fix and document visibility attributes for Clang, GCC and Windows.

Summary:
This patch fixes a number of problems with the visibility macros across GCC (on Unix) and Windows (DLL import/export semantics). All of the visibility macros are now documented under `DesignDocs/VisibilityMacros.rst`. Now I'll no longer forget the subtleties of each!

This patch adds two new visibility macros:

* `_LIBCPP_ENUM_VIS` for controlling the typeinfo of enum types. Only Clang supports this.
* `_LIBCPP_EXTERN_TEMPLATE_TYPE_VIS` for redefining visibility on explicit instantiation declarations. Clang and Windows require this.

After applying this patch GCC only emits one -Wattribute warning opposed to 30+.


Reviewers: mclow.lists, EricWF

Subscribers: beanz, mgorny, cfe-commits

Differential Revision: https://reviews.llvm.org/D24602

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@281673 91177308-0d34-0410-b5e6-96231b3b80d8

include/__string

@@ -701,7 +701,8 @@ __str_find_last_not_of(const _CharT *__p, _SizeT __sz,
 }
 
 template<class _Ptr>
-size_t _LIBCPP_INLINE_VISIBILITY __do_string_hash(_Ptr __p, _Ptr __e)
+inline _LIBCPP_INLINE_VISIBILITY
+size_t __do_string_hash(_Ptr __p, _Ptr __e)
 {
     typedef typename iterator_traits<_Ptr>::value_type value_type;
     return __murmur2_or_cityhash<size_t>()(__p, (__e-__p)*sizeof(value_type));