[libc++] Explicitly mark basic_string<...>::npos with default
visibility.

This ensures that the version compiled into the library isn't
accidentally hidden.

Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 2d8f23f571635c1fb983b40c4c2548716a5b65b6
diff --git a/include/string b/include/string
index c13cefc..0f84d87 100644
--- a/include/string
+++ b/include/string
@@ -4232,8 +4232,9 @@
 _LIBCPP_FUNC_VIS wstring to_wstring(long double __val);
 
 template<class _CharT, class _Traits, class _Allocator>
-    const typename basic_string<_CharT, _Traits, _Allocator>::size_type
-                   basic_string<_CharT, _Traits, _Allocator>::npos;
+_LIBCPP_FUNC_VIS
+const typename basic_string<_CharT, _Traits, _Allocator>::size_type
+               basic_string<_CharT, _Traits, _Allocator>::npos;
 
 template <class _CharT, class _Allocator>
 struct _LIBCPP_TEMPLATE_VIS