Reflection: use atoi instead of stoi

The Microsoft C++ library does not have std::stoi.  Opt to use std::atoi with
the C-string backing the std::string.  NFC.

Author: compnerd

include/swift/Reflection/MetadataSource.h

@@ -56,7 +56,7 @@ class MetadataSource {
     if (natural.empty())
       return false;
 
-    result = std::stoi(natural);
+    result = std::atoi(natural.c_str());
     return true;
   }