[torque]: Fix bugs reported by Peter Wong

* Empty string literals (e.g. "" and '') were not recognized a strings. This is
  now fixed.
* return statements without expressions (e.g. for functions with void return
  types) caused crashes.

Change-Id: Ied60f9abffca457a0d85c9e01e3795839fe777c9
Reviewed-on: https://chromium-review.googlesource.com/1042310
Commit-Queue: Daniel Clifford <danno@chromium.org>
Reviewed-by: Michael Stanton <mvstanton@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52997}

src/torque/Torque.g4

@@ -80,8 +80,8 @@ INCREMENT: '++';
 DECREMENT: '--';
 NOT: '!';
 
-STRING_LITERAL : ('"' ( ESCAPE | ~('"' | '\\' | '\n' | '\r') ) + '"')
-               | ('\'' ( ESCAPE | ~('"' | '\\' | '\n' | '\r') ) + '\'');
+STRING_LITERAL : ('"' ( ESCAPE | ~('"' | '\\' | '\n' | '\r') ) * '"')
+               | ('\'' ( ESCAPE | ~('"' | '\\' | '\n' | '\r') ) * '\'');
 fragment ESCAPE : '\\' ( '\'' | '\\' | '"' );
 
 IDENTIFIER  :   [A-Za-z][0-9A-Za-z_]* ;

src/torque/ast-generator.cc

@@ -287,8 +287,13 @@ antlrcpp::Any AstGenerator::visitExpressionStatement(
 
 antlrcpp::Any AstGenerator::visitReturnStatement(
     TorqueParser::ReturnStatementContext* context) {
-  return implicit_cast<Statement*>(RegisterNode(new ReturnStatement{
-      Pos(context), context->expression()->accept(this).as<Expression*>()}));
+  if (context->expression() != nullptr) {
+    return implicit_cast<Statement*>(RegisterNode(new ReturnStatement{
+        Pos(context), context->expression()->accept(this).as<Expression*>()}));
+  } else {
+    return implicit_cast<Statement*>(
+        RegisterNode(new ReturnStatement{Pos(context), {}}));
+  }
 }
 
 antlrcpp::Any AstGenerator::visitBreakStatement(