[llvm] [Demangle] Fix MSVC demangling for placeholder return types (#106178)

Properly demangle `_T` and `_P` return type manglings for MSVC 1920+.
Also added a unit test for `@` return type that is used when mangling
non-template auto placeholder return type function.

Tested the output against the undname shipped with MSVC 19.40.

Author: MaxEW707

llvm/include/llvm/Demangle/MicrosoftDemangleNodes.h

@@ -104,6 +104,8 @@ enum class PrimitiveKind {
   Double,
   Ldouble,
   Nullptr,
+  Auto,
+  DecltypeAuto,
 };
 
 enum class CharKind {

llvm/lib/Demangle/MicrosoftDemangle.cpp

@@ -2035,6 +2035,10 @@ Demangler::demanglePrimitiveType(std::string_view &MangledName) {
       return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char16);
     case 'U':
       return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char32);
+    case 'P':
+      return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Auto);
+    case 'T':
+      return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::DecltypeAuto);
     }
     break;
   }

llvm/lib/Demangle/MicrosoftDemangleNodes.cpp

@@ -149,6 +149,8 @@ void PrimitiveTypeNode::outputPre(OutputBuffer &OB, OutputFlags Flags) const {
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Double, "double");
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Ldouble, "long double");
     OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Nullptr, "std::nullptr_t");
+    OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, Auto, "auto");
+    OUTPUT_ENUM_CLASS_VALUE(PrimitiveKind, DecltypeAuto, "decltype(auto)");
   }
   outputQualifiers(OB, Quals, true, false);
 }

llvm/test/Demangle/ms-placeholder-return-type.test

@@ -0,0 +1,18 @@
+; RUN: llvm-undname < %s | FileCheck %s
+
+; CHECK-NOT: Invalid mangled name
+
+?TestNonTemplateAuto@@YA@XZ
+; CHECK: __cdecl TestNonTemplateAuto(void)
+
+??$AutoT@X@@YA?A_PXZ
+; CHECK: auto __cdecl AutoT<void>(void)
+
+??$AutoT@X@@YA?B_PXZ
+; CHECK: auto const __cdecl AutoT<void>(void)
+
+??$AutoT@X@@YA?A_TXZ
+; CHECK: decltype(auto) __cdecl AutoT<void>(void)
+
+??$AutoT@X@@YA?B_TXZ
+; CHECK: decltype(auto) const __cdecl AutoT<void>(void)