C++/WinRT fit and finish with support for OS and Xaml builds

[kennykerr]

This update aggregates a number of tweaks and fixes driven by OS and Xaml ingesting testing.

• Switch from pragma once to include guards to ease OS ingestion where we don’t control inclusion order.
• Re-enable header version mismatch detection.
• Remove stub projects that aren’t currently used (cppx, basewinrt, basex)
• Build cppwinrt on all platforms
• Minor code gen formatting improvements
• Re-enable cross-binary inheritance for Xaml
• Fix command line inconsistency between old cppwinrt and new cppwinrt
• Port support for JavaScript delegates that I added to C++/WinRT after the fork
• Port some warning silencing to ensure clean builds
• Enable type trait optimizations in Visual C++ (using std::disjunction_v rather than std::disjunction)
• Workaround for if constexpr parsing bugs in LKG7

[Scottj1s]