[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

diff --git a/include/string b/include/string
index b9e044e..39bda49 100644
--- a/include/string
+++ b/include/string
@@ -574,7 +574,7 @@
 #pragma warning( push )
 #pragma warning( disable: 4231 )
 #endif // _LIBCPP_MSVC
-_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_TYPE_VIS __basic_string_common<true>)
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS __basic_string_common<true>)
 #ifdef _LIBCPP_MSVC
 #pragma warning( pop )
 #endif // _LIBCPP_MSVC
@@ -1024,7 +1024,7 @@
     _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(size_type __pos1, size_type __n1, __self_view __sv) { return replace(__pos1, __n1, __sv.data(), __sv.size()); }
     basic_string& replace(size_type __pos1, size_type __n1, const basic_string& __str, size_type __pos2, size_type __n2=npos);
-    _LIBCPP_INLINE_VISIBILITY
+    inline _LIBCPP_INLINE_VISIBILITY
     basic_string& replace(size_type __pos1, size_type __n1, __self_view __sv, size_type __pos2, size_type __n2=npos);
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s, size_type __n2);
     basic_string& replace(size_type __pos, size_type __n1, const value_type* __s);
@@ -3910,8 +3910,8 @@
 }
 #endif
 
-_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_TYPE_VIS basic_string<char>)
-_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_TYPE_VIS basic_string<wchar_t>)
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_string<char>)
+_LIBCPP_EXTERN_TEMPLATE(class _LIBCPP_EXTERN_TEMPLATE_TYPE_VIS basic_string<wchar_t>)
 _LIBCPP_EXTERN_TEMPLATE(string operator+<char, char_traits<char>, allocator<char> >(char const*, string const&))
 
 _LIBCPP_END_NAMESPACE_STD