add an additional test case for generic attributes

gcc treats [[gnu:const]], [[gnu::__const]], and [[gnu:__const__]] as all being
equivalent.  Add an additional test case to ensure that we do not miss the last
case.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195982 91177308-0d34-0410-b5e6-96231b3b80d8