[ms-cxxabi] Move CodeGenVTables::needsVTTParameter to ItaniumCXXABI.

This function only makes sense there.  Eventually it should no longer
be part of the CGCXXABI interface, as it is an Itanium-specific detail.

Differential Revision: http://llvm-reviews.chandlerc.com/D821

llvm-svn: 185213