…cs in a namespace and all its descendant symbols

Fixes dotnet#486

This SuppressMessageAttribute feature has been requested for a very long time and still gets frequent customer requests to date.

The current "Namespace" target scope for SuppressMessageAttribute suppresses diagnostics only in nodes directly contained within the namespace, but in none of it's child symbols. This makes it very tedious to suppress certain analyzer diagnostics in entire namespace as one needs to add suppressions to individual types/namespaces within the namespace. This also creates a maintenance nightmare.

This PR adds a new target scope "NamespaceAndChildren" which suppresses diagnostics on the corresponding namespace and all its descendant symbols/members. Better naming suggestions for this scope name are welcome!