Make empty ids for all block types (#4502)

Adding empty IDs to all block types. This is primarily for
TypeBlockId::Empty, which allows me to disambiguate empty tuple types,
which I think might yield a SemIR readability improvement (PR
forthcoming). I'm splitting PRs so that the limited test impact here is
clear.

toolchain/check/pattern_match.cpp

@@ -126,7 +126,7 @@ auto MatchContext::DoWork(Context& context) -> SemIR::InstBlockId {
   while (!stack_.empty()) {
     EmitPatternMatch(context, stack_.pop_back_val());
   }
-  auto block_id = context.inst_blocks().AddOrEmpty(results_);
+  auto block_id = context.inst_blocks().Add(results_);
   results_.clear();
   return block_id;
 }