[MS ABI] Never reference dllimport'd vtables

Referencing a dllimported vtable is impossible in a constexpr
constructor.  It would be friendlier to C++ programmers if we
synthesized a copy of the vftable which referenced imported virtual
functions.  This would let us initialize the object in a way which
preserves both the intent to import functionality from another DLL while
also making constexpr work.

Differential Revision: http://reviews.llvm.org/D17061

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@260388 91177308-0d34-0410-b5e6-96231b3b80d8