cmIfCommand: Reject duplicate else() and misplaced elseif()

Closes: #14335

Source/cmIfCommand.h

@@ -21,6 +21,7 @@ public:
   cmIfFunctionBlocker()
   {
     this->HasRun = false;
+    this->ElseSeen = false;
     this->ScopeDepth = 0;
   }
   ~cmIfFunctionBlocker() CM_OVERRIDE {}
@@ -32,6 +33,7 @@ public:
   std::vector<cmListFileFunction> Functions;
   bool IsBlocking;
   bool HasRun;
+  bool ElseSeen;
   unsigned int ScopeDepth;
 };