VS: Do not compile C sources as WinRT (#15100)

The MSVC /ZW flag is valid only for C++ sources.  Whenever we enable
CompileAsWinRT for the whole target, disable it for all C sources.
Update the documentation of VS_WINRT_COMPONENT to drop the statement
about undefined behavior for non-C++ sources, because it is now
defined as expected.

Source/cmVisualStudio10TargetGenerator.h

@@ -134,6 +134,7 @@ private:
   std::string GUID;
   std::string Name;
   bool MSTools;
+  bool TargetCompileAsWinRT;
   cmGlobalVisualStudio10Generator* GlobalGenerator;
   cmGeneratedFileStream* BuildFileStream;
   cmLocalVisualStudio7Generator* LocalGenerator;