ILLink.LinkAttributes.xml file name is redundant

The name `ILLink.LinkAttributes.xml` is confusing and redundant. It doesn't appear to follow the other xml file names:

* ILLink.Descriptors.xml
* ILLink.Substitutions.xml

Why do we have "Link" twice in the name? Isn't the `ILLink` prefix enough to specify that this is for "linking"?

Code referencing this file makes it even more confusing. Take for example this MSBuild item/logic:

```xml

```

The double `LinkLink` looks like a copy-paste mistake and one of them should be removed.

cc @vitek-karas @marek-safar