`windows-rdl` / clang: CLSID constants

[riverar]

We're both aware of this task, but just creating an issue to take some notes.

[riverar]

We'll need to think about the mode clang is operating in and the effect it'll have parsing midl-generated headers. For example:

#ifdef __cplusplus
typedef class DiaSource DiaSource;
#else
typedef struct DiaSource DiaSource;
#endif /* __cplusplus */

...

EXTERN_C const CLSID CLSID_DiaSource;

#ifdef __cplusplus
class DECLSPEC_UUID("e6756135-1e65-4d17-8576-610761398c3c")
DiaSource;
#endif

[kennykerr]

So I think these should just be treated as GUID constants. Probably the simplest thing would be to convert class DECLSPEC_UUID("...") DiaSource; into const DiaSource: GUID = 0x.... if the C++ class is a forward declaration only.

[riverar]

Agreed. The annoying part is that on the C side, we have EXTERN_C const CLSID CLSID_DiaSource; which is populated at link time from the .lib file.

[kennykerr]

Right but we're not going to check lib files so we can just treat that as an undefined declaration to ignore I think.

[riverar]

Agree, should get handled via linker as normal (i.e., nag the user to link against diaguids.lib).