Don't dllimport/export destructor variants implemented by thunks.

MSVC doesn't export these functions, so trying to import them doesnt' work.
Also, don't let any dll attributes on the CXXDestructorDecl influence the
thunk's linkage -- they should always be linkonce_odr.

This takes care of the FIXME's for this in Nico's tests.

Differential Revision: http://reviews.llvm.org/D3930