On Windows, `GUID` is a currency type and (along with its various
typedefs) is used pervasively Windows offers `IsEqualGUID()` and
`UuidHash()` for comparing and hashing them, respectively, but
`IsEqualGUID()` is a macro in C mode and `UuidHash()` only provides a
16-bit hash. We should provide conformance to these protocols in the
WinSDK overlay to provide equivalent functionality in Swift.
This fixes modularization errors that arise when importing a C++ header that contains `#include <guiddef.h>`, which might hijack this header from the WinSDK module where it belongs.