cmake --build: Add support for driving Xcode workspaces

External tools may create a `.xcworkspace` directory next to the
`.xcodeproj` directory that CMake generates.  If a workspace exists,
drive the build through it instead.

Closes: #26958
Co-authored-by: Brad King <brad.king@kitware.com>

Source/cmGlobalXCodeGenerator.h

@@ -335,6 +335,7 @@ protected:
   BuildSystem XcodeBuildSystem = BuildSystem::One;
 
 private:
+  std::string GetAppleSpecificPlatformName();
   std::string const& GetXcodeBuildCommand();
   std::string FindXcodeBuildCommand();
   std::string XcodeBuildCommand;