[libcxx] Don't use dllimport for a static member in a template

This fixes clang warnings (that are treated as errors when running
the test suite):

libcxx/include/string:4409:59: error: definition of dllimport static field [-Werror,-Wdllimport-static-field-def]
               basic_string<_CharT, _Traits, _Allocator>::npos;

The warning is normally not visible as long as the libc++ headers
are treated as system headers.

The same construct is always an error in MSVC.

(One _LIBCPP_FUNC_VIS was added in
2d8f23f571635c1fb983b40c4c2548716a5b65b6, which broke DLL builds.
59919c4d6b6370da7133bbca0d31844e21646bb1 fixed this by adding another
_LIBCPP_FUNC_VIS on the declaration for consistency, but the underlying
issue remained, that one can't use dllimport here.)

Differential Revision: https://reviews.llvm.org/D97168

GitOrigin-RevId: c793f68d9b62e9813f17d438be5edd23f348b4b9
diff --git a/include/string b/include/string
index b052164..19bd22b 100644
--- a/include/string
+++ b/include/string
@@ -805,7 +805,7 @@
     __compressed_pair<__rep, allocator_type> __r_;
 
 public:
-    _LIBCPP_FUNC_VIS
+    _LIBCPP_TEMPLATE_DATA_VIS
     static const size_type npos = -1;
 
     _LIBCPP_INLINE_VISIBILITY basic_string()
@@ -4404,7 +4404,7 @@
 _LIBCPP_FUNC_VIS wstring to_wstring(long double __val);
 
 template<class _CharT, class _Traits, class _Allocator>
-_LIBCPP_FUNC_VIS
+_LIBCPP_TEMPLATE_DATA_VIS
 const typename basic_string<_CharT, _Traits, _Allocator>::size_type
                basic_string<_CharT, _Traits, _Allocator>::npos;