[MS ABI] Don't emit RTTI descriptors for dllimport vtables

A dllimport'd vtable always points one past the RTTI data, this means
that the initializer will never end up referencing the data.  Our
emission is a harmless waste.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260062 91177308-0d34-0410-b5e6-96231b3b80d8