cmake_host_system_information: Add undocumented VS_MSBUILD_COMMAND key

When using the Visual Studio generator for VS 10 or above,
offer this key to get the location of the MSBuild command
before the first `project()` or `enable_language()` command
has finished running.

This will be needed only by one of our own modules, so leave it
undocumented for now.

Source/cmCMakeHostSystemInformationCommand.cxx

@@ -27,6 +27,7 @@
 #ifdef _WIN32
 #  include "cmAlgorithms.h"
 #  include "cmGlobalGenerator.h"
+#  include "cmGlobalVisualStudio10Generator.h"
 #  include "cmGlobalVisualStudioVersionedGenerator.h"
 #  include "cmVSSetupHelper.h"
 #  define HAVE_VS_SETUP_HELPER
@@ -434,6 +435,12 @@ cm::optional<std::string> GetValue(cmExecutionStatus& status,
     }
   }
 
+  if (key == "VS_MSBUILD_COMMAND"_s && gg->IsVisualStudioAtLeast10()) {
+    cmGlobalVisualStudio10Generator* vs10gen =
+      static_cast<cmGlobalVisualStudio10Generator*>(gg);
+    return vs10gen->FindMSBuildCommandEarly(&status.GetMakefile());
+  }
+
   return {};
 }
 #endif