Projects with a nested Windows namespace (e.g., Microsoft::Windows) fail to compile

[Scottj1s]

Discovered with WinUI-PR pipeline run: update versions for CppWinRT, wil, BuildTools, .NET runtime

[Scottj1s]

Fixes build errors:

...\winrt\Microsoft.UI.Xaml.Markup.h(1278,53): Error C3083: 'Foundation': the symbol to the left of a '::' must be a type
...\winrt\Microsoft.UI.Xaml.Markup.h(1278,65): Error C2039: 'IInspectable': is not a member of 'winrt::Microsoft::Windows'
...\winrt\Microsoft.UI.Xaml.Markup.h(1291,65): Error C3083: 'Foundation': the symbol to the left of a '::' must be a type
...\winrt\Microsoft.UI.Xaml.Markup.h(1291,77): Error C2039: 'IInspectable': is not a member of 'winrt::Microsoft::Windows'

[sylveon]

Would you mind adding the fix for microsoft/WindowsAppSDK#1901 as well while we're on the topic of nested Windows namespaces?

[sylveon]

Also, maybe a unit test for this scenario would be desirable