[Windows Itanium][PS4] handle dllimport/export w.r.t vtables/rtti

The existing Windows Itanium patches for dllimport/export
behaviour w.r.t vtables/rtti can't be adopted for PS4 due to
backwards compatibility reasons (see comments on
https://reviews.llvm.org/D90299).

This commit adds our PS4 scheme for this to Clang.

Differential Revision: https://reviews.llvm.org/D93203