[cfe-dev] __uuidof() and declspec(uuid("...")) should be allowed on enumeration types

[Puetz Kevin A via cfe-dev]

Although not specifically mentioned in the documentation, MSVC accepts __uuidof(...) and declspec(uuid("...")) attributes on enumeration types in addition to structs/classes. This is meaningful, as such types *do* have associated UUIDs in ActiveX typelibs, and such attributes are included by default in the wrappers generated by their #import construct, so they are not particularly unusual.

clang currently rejects the declspec with a -Wignored-attributes warning, and errors on __uuidof() with "cannot call operator __uuidof on a type with no GUID" (because it rejected the uuid attribute, and therefore finds no value). This is causing problems for us while trying to use clang-tidy on a codebase that makes heavy use of ActiveX.

I believe I have found the relevant places to add this functionality, and attached are two patches adding this case to clang's implementation of these MS extensions. 
enum-uuidof-master.patch against r285994 (or actually the git mirror 80464680ce)
enum-uuidof-39.patch is the same, but based on release_39 at 282636. I don't seriously think this warrants a backport to 3.9, though I wouldn't object :-) However, I prepared the patch for our own use in the meantime, so I might as well include it. They differ only due to some reformatting of the error messages in include/clang/Basic/DiagnosticSemaKinds.td. 

Both include an update to test/Parser/MicrosoftExtensions.cpp to exercise the new functionality.

This is my first time contributing to LLVM, so if I've missed anything else needed to prepare this for review just let me know!

__uuidof: https://msdn.microsoft.com/en-us/library/zaah6a61.aspx
declspec(uuid("...")): https://msdn.microsoft.com/en-us/library/3b6wkewa.aspx
#import: https://msdn.microsoft.com/en-us/library/8etzzkb6.aspx
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