[Demangle] Support demangling Swift calling convention in MS demangler.

Previously, Clang was able to mangle the Swift calling
convention but 'MicrosoftDemangle.cpp' was not able to demangle it.

Reviewed By: compnerd, rnk

Differential Revision: https://reviews.llvm.org/D95053

include/llvm/Demangle/MicrosoftDemangleNodes.h

@@ -67,6 +67,7 @@ enum class CallingConv : uint8_t {
   Eabi,
   Vectorcall,
   Regcall,
+  Swift, // Clang-only
 };
 
 enum class ReferenceKind : uint8_t { None, LValueRef, RValueRef };

lib/Demangle/MicrosoftDemangle.cpp

@@ -1711,6 +1711,8 @@ CallingConv Demangler::demangleCallingConvention(StringView &MangledName) {
     return CallingConv::Eabi;
   case 'Q':
     return CallingConv::Vectorcall;
+  case 'S':
+    return CallingConv::Swift;
   }
 
   return CallingConv::None;

lib/Demangle/MicrosoftDemangleNodes.cpp

@@ -107,6 +107,9 @@ static void outputCallingConvention(OutputStream &OS, CallingConv CC) {
   case CallingConv::Clrcall:
     OS << "__clrcall";
     break;
+  case CallingConv::Swift:
+    OS << "__attribute__((__swiftcall__)) ";
+    break;
   default:
     break;
   }

test/Demangle/ms-mangle.test

@@ -338,6 +338,9 @@
 ?vector_func@@YQXXZ
 ; CHECK: void __vectorcall vector_func(void)
 
+?swift_func@@YSXXZ
+; CHECK: void __attribute__((__swiftcall__)) swift_func(void)
+
 ??$fn_tmpl@$1?extern_c_func@@YAXXZ@@YAXXZ
 ; CHECK: void __cdecl fn_tmpl<&void __cdecl extern_c_func(void)>(void)