[CUDA] Disallow overloading destructors.

Summary:
We'd attempted to allow this, but turns out we were doing a very bad
job.  :)

Making this work properly would be a giant change in clang.  For
example, we'd need to make CXXRecordDecl::getDestructor()
context-sensitive, because the destructor you end up with depends on
where you're calling it from.

For now (and hopefully for ever), just disallow overloading of
destructors in CUDA.

Reviewers: rsmith

Subscribers: cfe-commits, tra

Differential Revision: https://reviews.llvm.org/D24571

llvm-svn: 283120