CodeGen: Virtual dtor thunks shouldn't have this marked as 'returned'

The ARM ABI virtual destructor thunks cannot be marked as 'returned'
because they return undef.

git-svn-id: http://llvm.org/svn/llvm-project/cfe/trunk@221042 91177308-0d34-0410-b5e6-96231b3b80d8