MS ABI: #pragma vtordisp(0) only disables new vtordisps

Previously, it was believed that #pragma vtordisp(0) would prohibit the
generation of any and all vtordisps.

In actuality, it only disables the generation of additional vtordisps.

This fixes PR19413.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@206124 91177308-0d34-0410-b5e6-96231b3b80d8