[libc++] Fix extern template visibility for Windows

On Windows, marking an `extern template class` declaration as exported
actually forces an instantiation, which is not the desired behavior.
Instead, the actual explicit instantiations need to be exported.

Differential Revision: https://reviews.llvm.org/D24679

llvm-svn: 281925
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 190994e435884229ca058362e9974a5de4c41947
diff --git a/src/ios.cpp b/src/ios.cpp
index 23e3ee0..7c76ca2 100644
--- a/src/ios.cpp
+++ b/src/ios.cpp
@@ -25,19 +25,19 @@
 
 _LIBCPP_BEGIN_NAMESPACE_STD
 
-template class basic_ios<char>;
-template class basic_ios<wchar_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ios<char>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ios<wchar_t>;
 
-template class basic_streambuf<char>;
-template class basic_streambuf<wchar_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_streambuf<char>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_streambuf<wchar_t>;
 
-template class basic_istream<char>;
-template class basic_istream<wchar_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_istream<char>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_istream<wchar_t>;
 
-template class basic_ostream<char>;
-template class basic_ostream<wchar_t>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ostream<char>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_ostream<wchar_t>;
 
-template class basic_iostream<char>;
+template class _LIBCPP_CLASS_TEMPLATE_INSTANTIATION_VIS basic_iostream<char>;
 
 class _LIBCPP_HIDDEN __iostream_category
     : public __do_message