runtime: use _strdup on Windows

Windows prefers the `_strdup` extension over `strdup`. This avoids
unnecessary warnings when building the standard library.

include/swift/Demangling/TypeLookupError.h

@@ -84,7 +84,11 @@ private:
   static char *getCString(char *str) { return str; }
 
   static char *getCString(const std::string &str) {
+#if defined(_WIN32)
+    return _strdup(str.c_str());
+#else
     return strdup(str.c_str());
+#endif
   }
 
 public: