[SystemZ] Support Swift Calling Convention

Summary:
Port rL265480, rL264754, rL265997 and rL266252 to SystemZ, in order to enable the Swift port on the architecture. SwiftSelf and SwiftError are assigned to R10 and R9, respectively, which are normally callee-saved registers. For more information, see:

RFC: Implementing the Swift calling convention in LLVM and Clang
https://groups.google.com/forum/#!topic/llvm-dev/epDd2w93kZ0

Reviewers: kbarton, manmanren, rjmccall, uweigand

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D19414

llvm-svn: 267823