Take clang's isSwiftAsyncCCSupported into account

Author: aschwaighofer

lib/IRGen/IRGenModule.cpp

@@ -547,9 +547,12 @@ IRGenModule::IRGenModule(IRGenerator &irgen,
   // TODO: use "tinycc" on platforms that support it
   DefaultCC = SWIFT_DEFAULT_LLVM_CC;
   SwiftCC = llvm::CallingConv::Swift;
-  // TODO: Once clang support is merged this should also use
-  //  clangASTContext.getTargetInfo().isSwiftAsyncCCSupported()
-  SwiftAsyncCC = opts.UseAsyncLowering ? llvm::CallingConv::SwiftTail : SwiftCC;
+
+  bool isAsynCCSupported =
+      clangASTContext.getTargetInfo().isSwiftAsyncCCSupported();
+  SwiftAsyncCC = (opts.UseAsyncLowering && isAsynCCSupported)
+                     ? llvm::CallingConv::SwiftTail
+                     : SwiftCC;
 
   if (opts.DebugInfoLevel > IRGenDebugInfoLevel::None)
     DebugInfo = IRGenDebugInfo::createIRGenDebugInfo(IRGen.Opts, *CI, *this,