FindTypeByName always roots ExpandoObject (and is also undefined behavior)

[Sergio0694]

Describe the bug

This code in TypeNameSupport.FindTypeByName unconditionally roots ExpandoObject:

CsWinRT/src/WinRT.Runtime/TypeNameSupport.cs

Lines 105 to 112 in f01956b

	public static (Type type, int remaining) FindTypeByName(ReadOnlySpan<char> runtimeClassName)
	{
	// Assume that anonymous types are expando objects, whether declared 'dynamic' or not.
	// It may be necessary to detect otherwise and return System.Object.
	if (runtimeClassName.StartsWith("<>f__AnonymousType".AsSpan(), StringComparison.Ordinal))
	{
	return (typeof(System.Dynamic.ExpandoObject), 0);
	}

This shows up even in a minimal WinRT component with NAOT, not using anonymous/dynamic types at all:

Additionally, matching on "<>f__AnonymousType" is undefined behavior. The C# spec (nor Roslyn) guarantee that name prefix.