MS ABI: Pure virtual functions don't contribute to vtordisps

Usually, overriding a virtual function defined in a virtual base
required emission of a vtordisp slot in the record.  However no vtordisp
is needed if the overriding function is pure; it should be impossible to
observe the pure virtual method.

This fixes PR21046.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@218340 91177308-0d34-0410-b5e6-96231b3b80d8