View managers libraries should be able to autolink into an app's pch.h

If a view manager exposes a usercontrol or any IDL type, the xaml type info needs to be added to the consuming app's pch.h. This is standard for XAML and is a pain that developers often run into (if you don't manually add the include in the pch.h you get cryptic errors in a generated file XamlTypeInfo.g.cpp).

Proposal

1. app template's pch.h should include a new file, AutolinkedXamlTypeInfo.g.h
2. AutolinkedXamlTypeInfo.g.h would be generated via autolinking, inspecting the module and extracting metadata from the WinMD to determine which namespaces have any XAML-derived types. Then the contents of the AutolinkedXamlTypeInfo.g.h is simply a series of #include <winrt/NameOfTheDiscoveredNamespaces.h>

This however poses a problem, because the winmd is only generted after MIDL runs, so we need the build to autolink, but we need autolink to build. No good.

An alternate "good enough" approach, is to assume that any module's IDLs will be in the project's RootNamespace, and so we can use

#if __has_include(<winrt/TheProjectRootNamespace.h>)
#include <winrt/TheProjectRootNamespace.h>)
#endif

to include the cppwinrt-projected header for that namespace. If it has XAML types, great, we've solved the problem!
If it doesn't, it won't impact anything (since importing the header doesn't pollute the global namespace anyway).

Start by tracing the app template's pch.h and the build/autolinking steps around MIDL and WinMD generation. Compare the proposed AutolinkedXamlTypeInfo.g.h approach with the RootNamespace and __has_include alternative. Done means consuming apps automatically include the needed WinRT namespace header for view-manager XAML or IDL types without manual pch.h edits.