[torque] Use StringConstant when generating strings

Use StringConstant instead of NewStringFromAsciiChecked so strings are
deduplicated.

Change-Id: I0c5395be6d06caacd7d257b61bd2372da2fce427
Reviewed-on: https://chromium-review.googlesource.com/1025815Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Dan Elphick <delphick@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52763}

src/torque/implementation-visitor.cc

@@ -426,9 +426,8 @@ VisitResult ImplementationVisitor::Visit(NumberLiteralExpression* expr) {
 
 VisitResult ImplementationVisitor::Visit(StringLiteralExpression* expr) {
   std::string temp = GenerateNewTempVariable(GetTypeOracle().GetStringType());
-  source_out()
-      << "HeapConstant(isolate()->factory()->NewStringFromAsciiChecked(\""
-      << expr->literal.substr(1, expr->literal.size() - 2) << "\"));"
+  source_out() << "StringConstant(\""
+               << expr->literal.substr(1, expr->literal.size() - 2) << "\");"
                << std::endl;
   return VisitResult{GetTypeOracle().GetStringType(), temp};
 }