Add module definitions for string_view

llvm-svn: 288733
diff --git a/libcxx/include/string_view b/libcxx/include/string_view
index 60c0ad0..e66ac40 100644
--- a/libcxx/include/string_view
+++ b/libcxx/include/string_view
@@ -164,7 +164,10 @@
 #include <__config>
 
 #include <__string>
+#include <algorithm>
 #include <iterator>
+#include <limits>
+#include <stdexcept>
 #include <__debug>
 
 #if !defined(_LIBCPP_HAS_NO_PRAGMA_SYSTEM_HEADER)