[libc++] Mark string operator+ _LIBCPP_FUNC_VIS

It has an extern template instantiation declaration in the headers and a
corresponding instantiation definition in the library, so we must mark
it with _LIBCPP_FUNC_VIS to make it available outside the library.

This doesn't cause any ABI changes as-is since we don't build libc++
with hidden visibility (so the function is exported anyway). It's needed
for building libc++ with hidden visibility, however.

Clarify the Windows behavior for extern function templates while I'm
here, since this exercises that behavior.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@307966 91177308-0d34-0410-b5e6-96231b3b80d8

docs/DesignDocs/VisibilityMacros.rst

@@ -98,7 +98,8 @@ Visibility Macros
   explicit instantiations themselves are marked as exported. Note that this
   applies *only* to extern *class* templates. Extern *function* templates obey
   regular import/export semantics, and applying `dllexport` directly to the
-  extern template declaration is the correct thing to do for them.
+  extern template declaration (i.e. using `_LIBCPP_FUNC_VIS`) is the correct
+  thing to do for them.
 
 **_LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS**
   Mark the member functions, typeinfo, and vtable of an explicit instantiation

include/string

@@ -4006,7 +4006,7 @@ basic_string<_CharT, _Traits, _Allocator>::__subscriptable(const const_iterator*
 
 _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_EXTERN_TEMPLATE(_LIBCPP_FUNC_VIS string operator+<char, char_traits<char>, allocator<char> >(char const*, string const&))
 
 #if _LIBCPP_STD_VER > 11 
 // Literal suffixes for basic_string [basic.string.literals]