Platform: add a definition for std on Windows

This adds the c++ module for Windows to help further the C++ interop.

Author: compnerd

stdlib/public/Platform/ucrt.modulemap

@@ -13,6 +13,9 @@
 module ucrt [system] {
   module C {
     module complex {
+      /* disallow the header in C++ mode as it forwards to `ccomplex`.  */
+      requires !cplusplus
+
       header "complex.h"
       export *
     }