stdlib: use "C" style casts in the overlay shims

When this header is used to create a module, it is used in a "C" environment,
which means that we cannot use the C++ style casts.  Switch to the
old-style-cast.  If we need to, we can always use `-Wno-old-style-casts` to
silence the warnings in C++ mode.

Author: compnerd

stdlib/public/SwiftShims/LibcOverlayShims.h

@@ -99,7 +99,7 @@ static inline int _swift_stdlib_ioctlPtr(int fd, unsigned long int request, void
 #if defined(_WIN32) && !defined(__CYGWIN__)
 // Windows
 static inline int _swift_stdlib_open(const char *path, int oflag, mode_t mode) {
-  return _open(path, oflag, static_cast<int>(mode));
+  return _open(path, oflag, (int)mode);
 }
 
 #else