[SystemZ] Support Swift calling convention

Summary:
Port rL265324 to SystemZ to allow using the 'swiftcall' attribute on that architecture.

Depends on D19414.

Reviewers: kbarton, rjmccall, uweigand

Subscribers: cfe-commits

Differential Revision: http://reviews.llvm.org/D19432

llvm-svn: 267879