Refactor InstBlockStore's API, AddDefaultValue -> AddPlaceholder (#5166)

`AddDefaultValue` doesn't quite capture the intended semantics; it
should typically be replaced with an actual value when dealing with
control flows. Trying to indicate the "assign later" with
`AddPlaceholder`, mirroring `AddPlaceholderInst`.

Shifting the `protected` functionality on `BlockValueStore` so that it's
not providing functions just for `InstBlockStore` to use. Also hoping
that seeing the comments next to the function name makes them easier to
understand, whereas `using` buries that a little.

---------

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/check/inst_block_stack.cpp

@@ -29,7 +29,7 @@ auto InstBlockStack::PeekOrAdd(int depth) -> SemIR::InstBlockId {
   int index = id_stack_.size() - depth - 1;
   auto& slot = id_stack_[index];
   if (!slot.has_value()) {
-    slot = sem_ir_->inst_blocks().AddDefaultValue();
+    slot = sem_ir_->inst_blocks().AddPlaceholder();
   }
   return slot;
 }
@@ -42,7 +42,7 @@ auto InstBlockStack::Pop() -> SemIR::InstBlockId {
   // Finalize the block.
   if (!insts.empty() && id != SemIR::InstBlockId::Unreachable) {
     if (id.has_value()) {
-      sem_ir_->inst_blocks().Set(id, insts);
+      sem_ir_->inst_blocks().ReplacePlaceholder(id, insts);
     } else {
       id = sem_ir_->inst_blocks().Add(insts);
     }