[llvm] [Demangle] Fix MSVC demangling for placeholder return types

[MaxEW707]

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.

[zmodem]

lgtm

[Ryan-rsm-McKenzie]

Can't you just add these to PrimitiveKind and add the match cases to the end of demanglePrimitiveType? It already tries to parse _*:

llvm-project/llvm/lib/Demangle/MicrosoftDemangle.cpp

Lines 2016 to 2040 in 7c188ab

	case '_': {
	if (MangledName.empty()) {
	Error = true;
	return nullptr;
	}
	const char F = MangledName.front();
	MangledName.remove_prefix(1);
	switch (F) {
	case 'N':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Bool);
	case 'J':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Int64);
	case 'K':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Uint64);
	case 'W':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Wchar);
	case 'Q':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char8);
	case 'S':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char16);
	case 'U':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char32);
	}
	break;
	}

I don't see why you need to create and entirely new node type for something so simple.

[MaxEW707]

Can't you just add these to PrimitiveKind and add the match cases to the end of demanglePrimitiveType? It already tries to parse _*:

llvm-project/llvm/lib/Demangle/MicrosoftDemangle.cpp

Lines 2016 to 2040 in 7c188ab

	case '_': {
	if (MangledName.empty()) {
	Error = true;
	return nullptr;
	}
	const char F = MangledName.front();
	MangledName.remove_prefix(1);
	switch (F) {
	case 'N':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Bool);
	case 'J':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Int64);
	case 'K':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Uint64);
	case 'W':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Wchar);
	case 'Q':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char8);
	case 'S':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char16);
	case 'U':
	return Arena.alloc<PrimitiveTypeNode>(PrimitiveKind::Char32);
	}
	break;
	}

I don't see why you need to create and entirely new node type for something so simple.

Agreed. I assumed that wouldn't fly since pedantically auto and decltype(auto) aren't considered primitive types but I probably should have went with the easiest solution first.

I'll change the code to just add the two new cases to demanglePrimitiveType.

[MaxEW707]

@zmodem Let me know if the new changes in this PR are ok with you.