swift

4373bdd6 - Conditionally start using llvm::CallingConv::SwiftTail for async functions

Conditionally start using llvm::CallingConv::SwiftTail for async functions This is conditional on UseAsyncLowering and in the future should also be conditional on `clangTargetInfo.isSwiftAsyncCCSupported()` once that support is merged. Update tests to work either with swiftcc or swifttailcc.