Convert StructTypeField to a specific type. (#4492)

This converts `StructTypeField` from an instruction to a dedicated type,
with its own store. This had originated from discussing how
`.GetAs<SemIR::StructTypeField>` was more prevalent than for other
instructions, but is probably more interesting for the storage savings
(16 bytes StructTypeField + 4 byte LocId + 4 byte InstId -> 8 byte
StructTypeField).

Due to the different structure, these now have their own stack during
construction, reducing (but not eliminating) `args_type_info_stack_`
use-cases.

The test changes of different InstIds is expected because structs and
classes generate fewer instructions now. Other than that, results should
remain the same.

I'm generally trying to avoid unrelated cleanup here due to the PR size,
though I did scrutinize the `VerifyOnFinish` calls, adding one and
commenting others (putting them in member order because that's how I was
checking what was verified and what wasn't).

common/array_stack.h

@@ -84,6 +84,9 @@ class ArrayStack {
   // Returns the current number of values in all arrays.
   auto all_values_size() const -> size_t { return values_.size(); }
 
+  // Returns true if the stack has no arrays pushed.
+  auto empty() const -> bool { return array_offsets_.empty(); }
+
  private:
   // For each pushed array, the start index in elements_.
   llvm::SmallVector<int32_t> array_offsets_;