Do not create let/const Variables when TDZ is disabled.

Summary:
Ensure that all program declarations are handled as vars (which is almost what Hermes does)
when tdz is disabled, after which ```Variable::getObeysTDZ()``` can be implemented in terms of
the Variable's DeclKind.

Reviewed By: mattbfb

Differential Revision: D39337023

fbshipit-source-id: 2e11262bd9f74554564b2b06398776e75fffa351

include/hermes/IR/IR.h

@@ -900,11 +900,6 @@ class Variable : public Value {
     Const,
   };
 
-  /// Return true if this DeclKind needs to track TDZ.
-  static bool declKindNeedsTDZ(DeclKind dk) {
-    return dk != DeclKind::Var;
-  }
-
  private:
   Variable(const Variable &) = delete;
   void operator=(const Variable &) = delete;
@@ -918,9 +913,6 @@ class Variable : public Value {
   /// The scope that owns the variable.
   VariableScope *parent;
 
-  /// If true, this variable obeys the TDZ rules.
-  bool obeysTDZ_ = false;
-
  protected:
   explicit Variable(
       ValueKind k,
@@ -946,10 +938,7 @@ class Variable : public Value {
   }
 
   bool getObeysTDZ() const {
-    return obeysTDZ_;
-  }
-  void setObeysTDZ(bool value) {
-    obeysTDZ_ = value;
+    return declKind != DeclKind::Var;
   }
 
   /// Return the index of this variable in the function's variable list.

lib/IRGen/ESTreeIRGen.cpp

@@ -392,15 +392,12 @@ std::pair<Value *, bool> ESTreeIRGen::declareVariableOrGlobalProperty(
       vdc = Variable::DeclKind::Var;
     }
 
-    auto *var = Builder.createVariable(inFunc->getFunctionScope(), vdc, name);
-
-    // For "let" and "const" create the related TDZ flag.
-    if (Variable::declKindNeedsTDZ(vdc) &&
-        Mod->getContext().getCodeGenerationSettings().enableTDZ) {
-      var->setObeysTDZ(true);
+    if (!Mod->getContext().getCodeGenerationSettings().enableTDZ) {
+      // short-circuit all declarations to be Var. TDZ dies here.
+      vdc = Variable::DeclKind::Var;
     }
 
-    res = var;
+    res = Builder.createVariable(inFunc->getFunctionScope(), vdc, name);
   }
 
   // Register the variable in the scoped hash table.