Add enum:self for direct AST access in templates

Summary:
Add `enum:self` for direct AST access in templates. To this end pass the prototype database via `mstch_context` to mstch object ctors.

Demonstrate usage by updating `declare_enums.mustache`.

Before:
```
return "{{enum:name}}";
```

After:
```
return "{{enum:self.name}}";
```

Once we fully migrate templates, we'll be able to drop the `self` part which will give the same ergonomics as before but without the legacy mstch object layer.

To simplify the review `self` is only added to a single mstch object type in this diff, the rest will be added in a follow-up diff.

Reviewed By: praihan

Differential Revision: D71205072

fbshipit-source-id: 6db90dc5442e34bf7382264abe6a5c90f0ec3545

Author: vitaut

third-party/thrift/src/thrift/compiler/generate/mstch_objects.h

@@ -237,6 +237,8 @@ struct mstch_context : mstch_factories {
 
   node_metadata_cache metadata_cache;
 
+  const whisker::prototype_database* prototypes = nullptr;
+
   /**
    * Sets or erases the option with the given `key` depending on the
    * `condition`.
@@ -1196,6 +1198,7 @@ class mstch_enum : public mstch_base {
     register_methods(
         this,
         {
+            {"enum:self", &mstch_enum::self},
             {"enum:name", &mstch_enum::name},
             {"enum:values", &mstch_enum::values},
             {"enum:structured_annotations?",
@@ -1210,6 +1213,10 @@ class mstch_enum : public mstch_base {
         });
   }
 
+  whisker::object self() {
+    return whisker::object(whisker::native_handle<t_enum>(
+        whisker::manage_as_static(*enum_), context_.prototypes->of<t_enum>()));
+  }
   mstch::node name() { return enum_->get_name(); }
   mstch::node values();
   mstch::node unused_value() { return enum_->unused(); }

third-party/thrift/src/thrift/compiler/generate/t_mstch_generator.h

@@ -40,6 +40,7 @@ class t_mstch_generator : public t_whisker_generator {
       const std::map<std::string, std::string>& options) override {
     t_whisker_generator::process_options(options);
     mstch_context_.options = compiler_options();
+    mstch_context_.prototypes = render_state().prototypes.get();
   }
 
  protected:

third-party/thrift/src/thrift/compiler/generate/templates/cpp2/module_types_h/declare_enums.mustache

@@ -59,7 +59,7 @@ template <> struct TEnumTraits<::{{program:qualified_namespace}}::{{enum:cpp_nam
   static bool findValue(std::string_view name, type* out) noexcept;
 
   FOLLY_ERASE static std::string_view typeName() noexcept {
-    return "{{enum:name}}";
+    return "{{enum:self.name}}";
   }
 
   FOLLY_ERASE static constexpr std::string_view moduleName() noexcept {