Remove System.Collection.NonGeneric from SqlClient contract

[justinvp]

A new dependency on System.Collections.NonGeneric was recently added to System.Data.SqlClient in #6396 as part of exposing SqlBulkCopyColumnMappingCollection, which derives from CollectionBase.

This PR is made up of two commits:

1. The first commit removes the dependency from the contract, by changingSqlBulkCopyColumnMappingCollection to derive from IList directly instead of CollectionBase. This is consistent with other collections in CoreFX that derive from CollectionBase in the full framework (e.g. X509CertificateCollection [manual], XmlArrayItemAttributes [manual], XmlAnyElementAttributes [manual], etc.).
2. The second commit changes the actual SqlBulkCopyColumnMappingCollection implementation to implement IList directly instead of deriving from CollectionBase. Tests are included to verify the behavior remains the same as when the implementation came from CollectionBase (I also ran these tests on the full framework).

There are two additional related changes I'd like to address in separate PRs:

• Removing the dependency on System.Collections.NonGeneric from the SqlClient implementation. There are a few remaining uses of ArrayList and Hashtable in SqlClient that can be compatibly changed to no longer depend on the legacy collections. I plan to submit a separate PR that does this.
• This PR changes the implementation of SqlBulkCopyColumnMappingCollection to use List<object> as the backing collection, to maintain the same behavior as when it derived from CollectionBase. However, it'd make more sense to strongly type it as List<SqlBulkCopyColumnMapping> as this collection is only intended to hold SqlBulkCopyColumnMapping objects. I didn't do that as part of this PR because it would change the behavior of some of the non-generic IList members (for the better, IMO). For example, IList.Add(object) currently allows adding an object of any type, even though it should really throw if the object isn't SqlBulkCopyColumnMapping (we've made similar changes to other collections in CoreFX that previously derived from CollectionBase in the full framework like X509CertificateCollection, changing it to use List<X509Certificate> as the backing collection in Cleanup and harden X509Certificate collections #2824).

Fixes #6406.

cc: @saurabh500

[saurabh500]

@justinvp Thanks for the changes
I will go through them soon.

[saurabh500]

Why is the NonGenericList needed?
In the code the _list is being referenced using NonGenericList and _list member. Can we have this being referenced consistently?

[justinvp]

We need to call some of the explicitly implemented non-generic collection interface members on List<T> and having this property is a convenient way to do that. Otherwise, we'd have to cast the List<T> to the non-generic interfaces at the callsites, e.g.:

void ICollection.CopyTo(Array array, int index) => ((ICollection)_list).CopyTo(array, index);

We did the same thing in X509CertificateCollection to avoid the casts at the callsites.

corefx/src/System.Security.Cryptography.X509Certificates/src/System/Security/Cryptography/X509Certificates/X509CertificateCollection.cs

Lines 247 to 250 in c20fcbe

	private IList NonGenericList
	{
	get { return _list; }
	}

[saurabh500]

OK 👍

[saurabh500]

LGTM.