__uuidof() and declspec(uuid("...")) should be allowed on enumeration types

__uuidof() and declspec(uuid("...")) should be allowed on enumeration types

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,
this patch adds this case to clang’s implementation of these MS
extensions.  patch is against r285994 (or actually the git mirror
80464680ce).

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

Reviewers: aaron.ballman, majnemer, rnk

Differential Revision: https://reviews.llvm.org/D26846

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289567 91177308-0d34-0410-b5e6-96231b3b80d8