Platform: improve WinSDK modulemap to repair build
Platform: improve WinSDK modulemap to repair build

The WinSDK module failed to expose WinBase properly for reuse, which the
newer clang seems to object to (via an assertion).  Add a separate
module for WinBase which allows reuse of the definitions without causing
conflicts.  This caused some additional fallout requiring the creation
of the WinSVC submodule (service handling, part of security and
identity) and splitting up the legacy WinSock header from the WinSock2
module.  This allows building Foundation again on Windows.

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