Revert r199416, "MS ABI: Improve selection of an inheritance model"

It broke tests for targeting x86_64-pc-win32:

    Clang Tools :: clang-modernize/LoopConvert/array.cpp
    Clang :: CodeGenCXX/2010-05-10-Var-DbgInfo.cpp
    Clang :: CodeGenCXX/member-call-parens.cpp
    Clang :: CodeGenCXX/ptr-to-datamember.cpp
    Clang :: SemaTemplate/instantiate-function-1.cpp