Convert `CWinRT` to `@_implementationOnly`

Ideally, we would not need to have the CWinRT module exposed. Currently, use of SwiftWinRT will require the header exposed. However, we cannot currently mark the module as implementation only as we re-expose many of the ABI types (as typealiases).

Start by tracing how SwiftWinRT currently exposes the CWinRT module and re-exposes its ABI types as typealiases. Determine the module boundary that prevents @_implementationOnly, then verify that SwiftWinRT no longer requires the CWinRT header to be exposed while preserving the required ABI types.

Written by the indexing model from the issue text.