Replacing lowering vectors with FixedSizeValueStore (#5636)

Changes the vectors on `Lower::FileContext` to be `FixedSizeValueStore`
where possible, which we have several at this point.

This changes `FixedSizeValueStore` to prefer inferring the size from a
`ValueStore<IdT>`, which should make adding incorrect sizes harder. Note
I wasn't sure that adding a `size()` to `TypeStore` that returned
`insts().size()` would be good because it doesn't directly work that
way; `ConstantValueStore` would've also required more work since it
doesn't have access to that right now.

toolchain/parse/tree_and_subtrees.cpp

@@ -16,7 +16,8 @@ TreeAndSubtrees::TreeAndSubtrees(const Lex::TokenizedBuffer& tokens,
                                  const Tree& tree)
     : tokens_(&tokens),
       tree_(&tree),
-      subtree_sizes_(SubtreeSizeStore::MakeForOverwrite(tree_->size())) {
+      subtree_sizes_(
+          SubtreeSizeStore::MakeForOverwriteWithExplicitSize(tree_->size())) {
   // A stack of nodes which haven't yet been used as children.
   llvm::SmallVector<NodeId> size_stack;
   for (auto n : tree.postorder()) {