CodeGen: support SwiftError SwiftCC on Windows x64

Add support for passing SwiftError through a register on the Windows x64
calling convention.  This allows the use of swifterror attributes on
parameters which is used by the swift front end for the `Error`
parameter.  This partially enables building the swift standard library
for Windows x86_64.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@313791 91177308-0d34-0410-b5e6-96231b3b80d8
3 files changed