Parse Microsoft __declspec appearing after class body

MSVC applies these to the following declaration only if present, otherwise
silently ignores them whereas we'll issue a warning.

Handling differs from ordinary attributes appearing in the same place, so add a
Sema test to make sure we get it right.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@195577 91177308-0d34-0410-b5e6-96231b3b80d8