[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@185213 91177308-0d34-0410-b5e6-96231b3b80d8