[Clang] Introduce Swift async calling convention.

This change is intended as initial setup. The plan is to add
more semantic checks later. I plan to update the documentation
as more semantic checks are added (instead of documenting the
details up front). Most of the code closely mirrors that for
the Swift calling convention. Three places are marked as
[FIXME: swiftasynccc]; those will be addressed once the
corresponding convention is introduced in LLVM.

Reviewed By: rjmccall

Differential Revision: https://reviews.llvm.org/D95561

include/llvm/Demangle/MicrosoftDemangleNodes.h

@@ -68,6 +68,7 @@ enum class CallingConv : uint8_t {
   Vectorcall,
   Regcall,
   Swift,      // Clang-only
+  SwiftAsync, // Clang-only
 };
 
 enum class ReferenceKind : uint8_t { None, LValueRef, RValueRef };

lib/Demangle/MicrosoftDemangle.cpp

@@ -1713,6 +1713,8 @@ CallingConv Demangler::demangleCallingConvention(StringView &MangledName) {
     return CallingConv::Vectorcall;
   case 'S':
     return CallingConv::Swift;
+  case 'T':
+    return CallingConv::SwiftAsync;
   }
 
   return CallingConv::None;

lib/Demangle/MicrosoftDemangleNodes.cpp

@@ -110,6 +110,9 @@ static void outputCallingConvention(OutputStream &OS, CallingConv CC) {
   case CallingConv::Swift:
     OS << "__attribute__((__swiftcall__)) ";
     break;
+  case CallingConv::SwiftAsync:
+    OS << "__attribute__((__swiftasynccall__)) ";
+    break;
   default:
     break;
   }

lib/Transforms/IPO/MergeFunctions.cpp

@@ -709,7 +709,10 @@ void MergeFunctions::writeThunk(Function *F, Function *G) {
 
   CallInst *CI = Builder.CreateCall(F, Args);
   ReturnInst *RI = nullptr;
-  CI->setTailCall();
+  bool isSwiftTailCall = F->getCallingConv() == CallingConv::SwiftTail &&
+                         G->getCallingConv() == CallingConv::SwiftTail;
+  CI->setTailCallKind(isSwiftTailCall ? llvm::CallInst::TCK_MustTail
+                                      : llvm::CallInst::TCK_Tail);
   CI->setCallingConv(F->getCallingConv());
   CI->setAttributes(F->getAttributes());
   if (H->getReturnType()->isVoidTy()) {

test/Demangle/ms-mangle.test

@@ -341,6 +341,9 @@
 ?swift_func@@YSXXZ
 ; CHECK: void __attribute__((__swiftcall__)) swift_func(void)
 
+?swift_async_func@@YTXXZ
+; CHECK: void __attribute__((__swiftasynccall__)) swift_async_func(void)
+
 ??$fn_tmpl@$1?extern_c_func@@YAXXZ@@YAXXZ
 ; CHECK: void __cdecl fn_tmpl<&void __cdecl extern_c_func(void)>(void)