Add TypeDescriptionProviders to XLinq (dotnet/corefx#33082)

Add TypeDescriptionProviders to XLinq (dotnet/corefx#33082)

* Bring over XComponentModel code from desktop

* Format code and add License.

* Add TypeDescriptionProviders to XLinq

This adds TypeDescriptionProviders to XAttribute and XElement

To do this without introducing a TypeConverter dependency in XML I had to push
TypeDescriptionProviderAttribute down.

This still causes TypeConverter to gain a dependency on XML, but right now that's the direction
we've been heading with TypeConverter.

I had to use the string overload of TypeDescriptionProviderAttribute in order to have a soft
dependency on TypeConverter.

* Format XComponentModel source for access / naming

* Remove TypeDescriptionProvider from reference assemblies

I decided to remove this from the reference assemblies.  None  of the usages I found would require
presense in the reference assemblies and it would expose too much internal surface which we might
want to change in the future (type name, and assembly).

Commit migrated from https://github.com/dotnet/corefx/commit/a01c44c78d37267c4e30cb1d647376e103557493