[cxx-interop] Modularize xsmf_control on Windows

This adds `xsmf_control.h` into the STL modulemap on Windows.

Fixes the following compiler error emitted by Clang when trying to use `std::variant` from Swift with C++ interop:
```
...\MSVC\14.42.34433\include\variant:923:25: error: missing '#include <xsmf_control.h>'; '_Non_trivial_move_assign' must be defined before it is used
```

Author: egorzhdan

stdlib/public/Platform/vcruntime.modulemap

@@ -757,6 +757,11 @@ module std [system] {
       export *
     }
 
+    explicit module xsmf_control {
+      header "xsmf_control.h"
+      export *
+    }
+
     explicit module xstring {
       header "xstring"
       export *