cmGlobalGenerator: Do not persist alias targets across configures

In `ccmake` a single global generator instance may be used for multiple
configure step runs.  The `cmGlobalGenerator::ClearGeneratorMembers`
method is supposed to clear global state that is specific to each
configure run but forgot to clear alias targets.

Fixes: #19457

Source/cmGlobalGenerator.cxx

@@ -1681,6 +1681,7 @@ void cmGlobalGenerator::ClearGeneratorMembers()
   cmDeleteAll(this->LocalGenerators);
   this->LocalGenerators.clear();
 
+  this->AliasTargets.clear();
   this->ExportSets.clear();
   this->TargetDependencies.clear();
   this->TargetSearchIndex.clear();