[torque] annotate generated source with original source positions

Bug: v8:7793
Change-Id: Ifb03938e15307910ef25b2b95c32fe69bfec1441
Reviewed-on: https://chromium-review.googlesource.com/1124458Reviewed-by: Daniel Clifford <danno@chromium.org>
Commit-Queue: Tobias Tebbi <tebbi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54169}

src/torque/implementation-visitor.cc

@@ -27,6 +27,8 @@ VisitResult ImplementationVisitor::Visit(Expression* expr) {
 
 const Type* ImplementationVisitor::Visit(Statement* stmt) {
   CurrentSourcePosition::Scope scope(stmt->pos);
+  GenerateIndent();
+  source_out() << "// " << CurrentPositionAsString() << "\n";
   switch (stmt->kind) {
 #define ENUM_ITEM(name)        \
   case AstNode::Kind::k##name: \