Optimize finding CustomAttribute by a full name

[AlanLiu90]

This PR improves CustomAttributeCollection.Find, CustomAttributeCollection.FindAll and CustomAttributeCollection.RemoveAll by comparing the suffix of the full name first. It can avoid calling CustomAttribute.TypeFullName in most of the time.

In a workflow I'm using currently, this PR can reduce the total running time by 29%.

[wtfsck]

The args (string fullName) could be null before but will now throw if null so we need a null check at the beginning of the methods now (return early if null).

[AlanLiu90]

I updated the code to check if fullName is null in these methods.

[wtfsck]

Thanks, merged!