libcxx: Define set_unexpected, _get_unexpected and __uncaught_exceptions without dllimport.

[pcc]

Summary

It turns out that the MSVC headers define these functions without
dllimport even when compiling with /MD. This change fixes the resulting
compile-time error.

[smeenai]

Seems like an oversight on their part, since these functions will be imported from VCRUNTIME*.dll for /MD. Oh well. LGTM.

libcxx/src/support/runtime/exception_msvc.ipp

 
 extern "C" {
 typedef void (__cdecl* terminate_handler)();
 _LIBCPP_CRT_FUNC terminate_handler __cdecl set_terminate(
     terminate_handler _NewTerminateHandler) throw();
 _LIBCPP_CRT_FUNC terminate_handler __cdecl _get_terminate();
 
 typedef void (__cdecl* unexpected_handler)();
-_LIBCPP_CRT_FUNC unexpected_handler __cdecl set_unexpected(
+unexpected_handler __cdecl set_unexpected(
     unexpected_handler _NewUnexpectedHandler) throw();
-_LIBCPP_CRT_FUNC unexpected_handler __cdecl _get_unexpected();
+unexpected_handler __cdecl _get_unexpected();
 
-_LIBCPP_CRT_FUNC int __cdecl __uncaught_exceptions();
+int __cdecl __uncaught_exceptions();
 }
 
 namespace std {
 
 unexpected_handler
 set_unexpected(unexpected_handler func) _NOEXCEPT {
   return ::set_unexpected(func);
 }