Consider naming for ILLink.Shared project

The shared project allows to share common code in a seamless way for Analyzer and linker in the dotnet/linker project, but is also code that is common in the NativeAOT repo. Although NativeAOT has to have a copy on its own for the shared project they can take advantage of the code inside the project. PR https://github.com/dotnet/runtime/pull/63230 adds the ILLink.Shared project to NativeAOT to have common diagnostics with Linker and Analyzer. One of the concerns is the name of this project, for dotnet/linker is accurate but it can confuse users of the NativeAOT area

The new name should cover the uses of the project which currently is diagnostics, but eventually will contain other shared code

cc @vitek-karas @sbomer @agocke @MichalStrehovsky