Support generic type parameter when created with TypeBuilder

[steveharter]

Fixes #112155

There may be other related edge cases related to "signature types"; this PR only fixes the case mentioned.

[jkotas]

Suggested change

	private static void MyGenericValueTypeMethod<T>() where T : struct { }
	private static void MyGenericRefTypeMethod<T>() where T : class { }

[jkotas]

Move up before Type et = t.GenericTypeArguments[0]; line and change to Assert.False.

[steveharter]

In this case, I wanted to make sure the new code that was added for IsValueType\IsEnum throws when the generic parameter is a signature type.

If I compare t with t.IsValueType that is just testing normal reflection and will return true for Span<sigtype> and false for the List<sigtype>.

[jkotas]

Type.MakeGenericSignatureType(typeof(List<>), typeof(int)).GetGenericArguments()[0] is going to be typeof(int), so this test does not seem to be actually testing IsValueType property on signature types.

Should this rather be something like:

yield return new object[] { Type.MakeGenericSignatureType(typeof(List<>), typeof(int)), false };
yield return new object[] { Type.MakeGenericSignatureType(typeof(KeyValuePair<,>), typeof(string), typeof(object)), true };

?

[steveharter]

Correct this is not verifying signature types but is verifying that when a non-signature type is used as a generic parameter then IsValueType works as expected. The test with GetGenericArguments() is done in MakeSignatureConstructedGenericType() as mentioned in the comment.

I'm not sure if your feedback here came before or after I pushed a new commit that changed this and added the comment.

[jkotas]

Dtto