Add undocumented CMake language means to find VS 2017

Add a query to the `cmake_host_system_information` command to get
the location of a VS 2017 installation.  Leave it undocumented and
for internal use for now.

Source/cmCMakeHostSystemInformationCommand.cxx

@@ -7,6 +7,12 @@
 #include "cmMakefile.h"
 #include "cmsys/SystemInformation.hxx"
 
+#if defined(_WIN32)
+#include "cmSystemTools.h"
+#include "cmVSSetupHelper.h"
+#define HAVE_VS_SETUP_HELPER
+#endif
+
 class cmExecutionStatus;
 
 // cmCMakeHostSystemInformation
@@ -70,6 +76,13 @@ bool cmCMakeHostSystemInformationCommand::GetValue(
     value = this->ValueToString(info.GetTotalPhysicalMemory());
   } else if (key == "AVAILABLE_PHYSICAL_MEMORY") {
     value = this->ValueToString(info.GetAvailablePhysicalMemory());
+#ifdef HAVE_VS_SETUP_HELPER
+  } else if (key == "VS_15_DIR") {
+    cmVSSetupAPIHelper vsSetupAPIHelper;
+    if (vsSetupAPIHelper.GetVSInstanceInfo(value)) {
+      cmSystemTools::ConvertToUnixSlashes(value);
+    }
+#endif
   } else {
     std::string e = "does not recognize <key> " + key;
     this->SetError(e);