X86: treat SwiftCC as Win64_CC on Win64

The Swift CC is identical to Win64 CC with the exception of swift error
being passed in r12 which is a CSR.  However, since this calling
convention is only used in swift -> swift code, it does not impact
interoperability and can be treated entirely as Win64 CC.  We would
previously incorrectly lower the frame setup as we did not treat the
frame as conforming to Win64 specifications.

llvm-svn: 313813