LibJS: Convert resolve_this_binding() to ThrowCompletionOr

Also add spec comments.

Userland/Libraries/LibJS/AST.cpp

@@ -2252,7 +2252,7 @@ Value SpreadExpression::execute(Interpreter& interpreter, GlobalObject& global_o
 Value ThisExpression::execute(Interpreter& interpreter, GlobalObject& global_object) const
 {
     InterpreterNodeScope node_scope { interpreter, *this };
-    return interpreter.vm().resolve_this_binding(global_object);
+    return TRY_OR_DISCARD(interpreter.vm().resolve_this_binding(global_object));
 }
 
 void ThisExpression::dump(int indent) const

Userland/Libraries/LibJS/Bytecode/Op.cpp

@@ -315,7 +315,13 @@ void Jump::execute_impl(Bytecode::Interpreter& interpreter) const
 
 void ResolveThisBinding::execute_impl(Bytecode::Interpreter& interpreter) const
 {
-    interpreter.accumulator() = interpreter.vm().resolve_this_binding(interpreter.global_object());
+    auto this_binding_or_error = interpreter.vm().resolve_this_binding(interpreter.global_object());
+    if (this_binding_or_error.is_throw_completion()) {
+        interpreter.vm().throw_exception(interpreter.global_object(), this_binding_or_error.release_error().value());
+        return;
+    }
+
+    interpreter.accumulator() = this_binding_or_error.release_value();
 }
 
 void Jump::replace_references_impl(BasicBlock const& from, BasicBlock const& to)

Userland/Libraries/LibJS/Runtime/VM.cpp

@@ -503,10 +503,12 @@ void VM::throw_exception(Exception& exception)
 }
 
 // 9.4.4 ResolveThisBinding ( ), https://tc39.es/ecma262/#sec-resolvethisbinding
-Value VM::resolve_this_binding(GlobalObject& global_object)
+ThrowCompletionOr<Value> VM::resolve_this_binding(GlobalObject& global_object)
 {
+    // 1. Let envRec be GetThisEnvironment().
     auto& environment = get_this_environment(*this);
-    return TRY_OR_DISCARD(environment.get_this_binding(global_object));
+    // 2. Return ? envRec.GetThisBinding().
+    return TRY(environment.get_this_binding(global_object));
 }
 
 String VM::join_arguments(size_t start_index) const

Userland/Libraries/LibJS/Runtime/VM.h

@@ -155,7 +155,7 @@ public:
         return running_execution_context().this_value;
     }
 
-    Value resolve_this_binding(GlobalObject&);
+    ThrowCompletionOr<Value> resolve_this_binding(GlobalObject&);
 
     Value last_value() const { return m_last_value; }
     void set_last_value(Badge<Bytecode::Interpreter>, Value value) { m_last_value = value; }