Don't allow dllimport/export on classes with internal linkage (PR21399)

Trying to import or export such classes doesn't make sense, and Clang
would assert trying to export vtables for them.

This is consistent with how we treat functions with internal linkage,
but it is stricter than MSVC so we may have to back down if it breaks
real code.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221160 91177308-0d34-0410-b5e6-96231b3b80d8