Initialize IMPORTED GLOBAL targets on reconfigure (#13702)

Since commit ca39c5cd (Optionally allow IMPORTED targets to be globally
visible, 2012-01-25) cmGlobalGenerator has a second member that tracks
targets with global scope.  We must initialize the new 'ImportedTargets'
member wherever the old 'TotalTargets' member is initialized.  Without
this initialization the ImportedTargets member is left with dangling
pointers during a same-process re-configuration.

Source/cmGlobalGenerator.cxx

@@ -817,6 +817,7 @@ void cmGlobalGenerator::Configure()
   this->LocalGenerators.clear();
   this->TargetDependencies.clear();
   this->TotalTargets.clear();
+  this->ImportedTargets.clear();
   this->LocalGeneratorToTargetMap.clear();
   this->ProjectMap.clear();
   this->RuleHashes.clear();