SymbolDisplayVisitor doesn't correctly classify function pointer calling conventions

[Youssef1313]

There are four calling conventions that are special-cased and hard coded in the compiler implementation. They don't necessarily represent classes.

roslyn/src/Compilers/CSharp/Portable/SymbolDisplay/SymbolDisplayVisitor.Members.cs

Lines 593 to 604 in eba055a

	case SignatureCallingConvention.CDecl:
	builder.Add(CreatePart(SymbolDisplayPartKind.ClassName, symbol, "Cdecl"));
	break;
	case SignatureCallingConvention.StdCall:
	builder.Add(CreatePart(SymbolDisplayPartKind.ClassName, symbol, "Stdcall"));
	break;
	case SignatureCallingConvention.ThisCall:
	builder.Add(CreatePart(SymbolDisplayPartKind.ClassName, symbol, "Thiscall"));
	break;
	case SignatureCallingConvention.FastCall:
	builder.Add(CreatePart(SymbolDisplayPartKind.ClassName, symbol, "Fastcall"));
	break;

roslyn/src/Compilers/CSharp/Portable/Symbols/FunctionPointers/FunctionPointerMethodSymbol.cs

Lines 147 to 154 in 7ba92d6

	case { CallingConventions: { Count: 1 } specifiers }:
	return specifiers[0].Name switch
	{
	// Special identifiers cases
	{ ValueText: "Cdecl" } => CallingConvention.CDecl,
	{ ValueText: "Stdcall" } => CallingConvention.Standard,
	{ ValueText: "Thiscall" } => CallingConvention.ThisCall,
	{ ValueText: "Fastcall" } => CallingConvention.FastCall,

While the BCL have corresponding classes, e.g, System.Runtime.CompilerServices.CallConvCdecl. The language rules don't seem to require these classes to exist, so IMO these are more like keywords not class names.

This issue is opened as the first step towards getting #59052.

[Youssef1313]

Closing per the discussion with @333fred