Fix condition in the runtime from _MSC_VER to _WIN32

Author: hughbe

stdlib/public/SwiftShims/LibcShims.h

@@ -35,7 +35,7 @@ namespace swift { extern "C" {
 // the current platform in the runtime code.
 #if defined(__linux__) && defined (__arm__) && !defined(__android__)
 typedef           int __swift_ssize_t;
-#elif defined(_MSC_VER)
+#elif defined(_WIN32)
 #if defined(_M_ARM) || defined(_M_IX86)
 typedef           int __swift_ssize_t;
 #elif defined(_M_X64)