ARM, AArch64: support `__attribute__((__swiftcall__))`

Support the Swift calling convention on Windows ARM and AArch64.  Both
of these conform to the AAPCS, AAPCS64 calling convention, and LLVM has
been adjusted to account for the register usage.  Ensure that the
frontend passes this into the backend.  This allows the swift runtime to
be built for Windows.

llvm-svn: 348454