Don't use __uuidof without GUID

[oldnewthing]

If GUID is not defined, then don't breathe the name of the __uuidof intrinsic, because that would instantiate an undefined structure. clang is stricter about this than msvc, but fix both just in case.

Fixes #1179

[oldnewthing]

The hardest part about this fix is getting clang installed and consumed. I had to hack it because somebody is adding /external:W4 to the command line options, which clang-cl then complains about:

clang-cl : error : no such file or directory: '/external:W4'

Also, the Directory.build.props is hard-coded to C:\Program Files\LLVM which means it doesn't accept the clang that comes with VS (in C:\Program Files (x86)\Microsoft Visual Studio\yyyy\edition\VC\Tools\Llvm\x64\bin). Once I have a fix for these issues, I'll create another PR.

[kennykerr]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 1 pipeline(s).

[sylveon]

It should be possible to use the LLVM platform toolset to build with clang, instead of hardcoding a path.