Fix signature parsing edge cases

[sbomer]

The signature parser (#1197) adapted from Roslyn's documentation comment id parser has some edge cases in which malformed inputs are still accepted. We mostly match the Roslyn behavior in these cases. Once dotnet/roslyn#44315 is fixed we should update our implementation to match.

[sbomer]

For future reference: @eerhardt mentioned he didn't get warnings when using:

[DynamicDependency("DefaultIfEmpty`1(System.Collections.Generic.IEnumerable{``0})", typeof(Enumerable))]

for a method

public static IEnumerable<TSource?> DefaultIfEmpty<TSource>(this IEnumerable<TSource> source);

where the `1 should be ``1 to denote the number of method generic parameters rather than the number of type generic parameters. This looks like another case where the parser is overly accepting.

[eerhardt]

where the `1 should be ``1

The `1 above isn't a reference to a generic type parameter. It is the number of generic arguments this method takes, right?

[sbomer]

Yes - edited for clarity.