Factor out Itanium C++ ABI support

by Chip for LLVM Compiler Infrastructure

Currently, Clang, like GCC, only supports the Itanium C++ ABI. This is only one of many C++ ABIs in existence. Other C++ ABIs include the various vendor-specific ABIs used on various UNIXes, and the Microsoft Visual C++ ABI. This project proposes to introduce an interface to the IRGen library to support multiple C++ ABis, and to factor Itanium C++ ABI support to use this new interface.