Associating GUID (or UUID) with types

[jsmall-zzz]

• Added macros SLANG_COM_INTERFACE and SLANG_CLASS_GUID that can be used to setup a guid in interface/class
• Made the #define SLANG_UUID_ just use the other macros
• Removed the places where guids where defined in C++ files as no longer required

This wasn't a new idea - it's covered in #1645

The main observation is that whilst it's not possible to define a Guid directly within an interface, without it being implemented in a C++ file, it is possible to do so within a function. If we inline the function we get much the same effect - although there are no assurances around what the pointers will be across files.

I tested on compiler explorer, and all major compiler do correctly inline and behave as if it is a global in release.

I did not replace in gfx until deemed appropriate and since lots of changes there.

[tangent-vector]

This looks pretty good, although I suppose there is a chance that a client would rather embed the GUIDs/UUIDs (I'm never totally clear on the distinction, but I know that UUID tends to be used outside of the Microsoft ecosystem) into their application via their own approach rather than rely on the accessor functions generated here. We can probably cross that bridge when we come to it, since the convenience here is high.

One other thing is that we could consider having another case of as<T> for ISlangUnknown-derived types that calls queryInterface with T::getTypeGuid() to streamline some of the casting sites. Not a real priority any time soon, though.