Convert remaining type-value InstId fields to TypeInstId (#5294)

After #5280 there are a few more typed instructions that have an `InstId
type_inst_id` that always holds a type value. These are converted to
`TypeInstId` to encode this fact in the type system. The
`ConvertAggregateElement()` function in convert.cpp is now able to
receive `TypeInstId` for a couple arguments as well.

Additionally, the `type_inst_id` field of `StructTypeField` is made into
a `TypeInstId`.

The `TupleType::elements_id` is renamed to `TupleType::type_elements_id`
to try record the fact that it's an InstBlock of type value
instructions. We don't introduce a TypeInstBlockId at this time, but it
might be nice to make blocks of TypeInstIds in the future.

To assist in working with a block of InstId that are type values, two
additional helpers are added to the TypeStore:
- GetBlockAsTypeInstIds which turns an `ArrayRef<InstId>` into a range
of `TypeInstId`
- GetBlockAsTypeIds which turns an `ArrayRef<InstId>` into a range of
`TypeId`

We use these helpers in places that iterate over the
`TupleType::type_elements_id`.

toolchain/check/pattern_match.cpp

@@ -528,17 +528,16 @@ auto MatchContext::DoEmitPatternMatch(Context& context,
   auto tuple_type =
       context.types().GetAs<SemIR::TupleType>(tuple_pattern.type_id);
   auto element_type_inst_ids =
-      context.inst_blocks().Get(tuple_type.elements_id);
+      context.inst_blocks().Get(tuple_type.type_elements_id);
   llvm::SmallVector<SemIR::InstId> subscrutinee_ids;
   subscrutinee_ids.reserve(element_type_inst_ids.size());
-  for (auto [i, element_type_inst_id] :
-       llvm::enumerate(element_type_inst_ids)) {
-    subscrutinee_ids.push_back(AddInst<SemIR::TupleAccess>(
-        context, scrutinee.loc_id,
-        {.type_id =
-             context.types().GetTypeIdForTypeInstId(element_type_inst_id),
-         .tuple_id = entry.scrutinee_id,
-         .index = SemIR::ElementIndex(i)}));
+  for (auto [i, element_type_id] : llvm::enumerate(
+           context.types().GetBlockAsTypeIds(element_type_inst_ids))) {
+    subscrutinee_ids.push_back(
+        AddInst<SemIR::TupleAccess>(context, scrutinee.loc_id,
+                                    {.type_id = element_type_id,
+                                     .tuple_id = entry.scrutinee_id,
+                                     .index = SemIR::ElementIndex(i)}));
   }
   add_all_subscrutinees(subscrutinee_ids);
 }