Fix infinite loop in SigComparer.GetHashCode(TypeSig)

[wwh1004]

Fix #427
In code:

static void Test<T>(Dictionary<T, T> dict) { string.Concat(dict); }

MethodSig of 'string.Concat' is 'string (IEnumerable<KeyPairValue<T, T>>)' and dnlib will repeat substitute one of 'T' in 'KeyPairValue<T, T>' with ''KeyPairValue<T, T>'' itself, so there is an infinite loop. I add a parameter to disable substituting when get hash code for 'GenericInstSig.GenericArguments'.

About deleted code:

if (SubstituteGenericParameters) {
	InitializeGenericArguments();
	genericArguments.PushTypeArgs(gia.GenericArguments);
	hash += GetHashCode(gia.GenericType);
	genericArguments.PopTypeArgs();
}

GenericType is always TypeDef or TypeRef so it makes no sense to substitute generic parameters.

[CreateAndInject]

Nice, but can you ensure this?

[wtfsck]

Thanks for the PR, will check later when I have more time.

[wwh1004]

Still contains some problems. WIP

[wwh1004]

Currently it will not resolve 'T' which is already resolved. In reflection apis, ga is always closed instantiation. So we don't need to substitute an already resolved ga.

[wwh1004]

In theory, these changes will not affect SigComparer's handling of signatures which can be fully represented by System.Reflection types, since System.Reflection types do not allow open instantiation. It should ensure that the original code behavior is not changed. Namely it just fixes this issue without side effects.

[wtfsck]

Merged now, thanks!