tbv2: account for duplicate types when emitting name providers

ClangTypeParser has emitted a duplicate type for `std::allocatr<int8_t>`.
Rather than fixing this, add the same check the compiler will do for the
duplicate templates that `addNames` emits. That is, `template<>
NameProvider<Foo>` will collide if `Foo` is used twice. We can do this by
adding a set of these strings for now. If this shows up regularly it will
likely make sense to deduplicate the type graph with a deduplication pass.

Test plan:
- Fixes the issue in prod. This change is quite logical.

oi/CodeGen.cpp

@@ -213,9 +213,13 @@ template <typename T>
 struct NameProvider {};
 )";
 
+  // TODO: stop types being duplicated at this point and remove this check
+  std::unordered_set<std::string_view> emittedTypes;
   for (const Type& t : typeGraph.finalTypes) {
     if (dynamic_cast<const Typedef*>(&t))
       continue;
+    if (!emittedTypes.emplace(t.name()).second)
+      continue;
 
     code += "template <> struct NameProvider<";
     code += t.name();