Portability fix: add missing includes and static_casts. Reviewed as https://reviews.llvm.org/D55777. Thanks to Andrey Maksimov for the patch.

llvm-svn: 349566
diff --git a/libcxx/test/std/strings/string.conversions/to_wstring.pass.cpp b/libcxx/test/std/strings/string.conversions/to_wstring.pass.cpp
index 281aa1a..2208ec5 100644
--- a/libcxx/test/std/strings/string.conversions/to_wstring.pass.cpp
+++ b/libcxx/test/std/strings/string.conversions/to_wstring.pass.cpp
@@ -19,6 +19,7 @@
 // wstring to_wstring(double val);
 // wstring to_wstring(long double val);
 
+#include <limits>
 #include <string>
 #include <cassert>
 #include <sstream>