__declspec(uuid) is only allowed on a class according to MSDN; this makes the...

__declspec(uuid) is only allowed on a class according to MSDN; this makes the semantic checking consistent with what the attribute specifies in Attr.td. Also adds a test case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195579 91177308-0d34-0410-b5e6-96231b3b80d8