There is no such thing as __declspec(ms_struct), this is a GNU attribute. Switched the attribute to have the proper spelling, gave it a subject, updated the warning to be more accurate, and updated the test case as appropriate.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195277 91177308-0d34-0410-b5e6-96231b3b80d8