Make function definitions allocate the body more lazily. (#2557)

Lazy allocation means that we can use a single node block for _all_ empty node blocks. The change in timing for when the definition node is emitted shouldn't affect semantic correctness; the signature is already present for recursive calls.

toolchain/semantics/semantics_node_block_stack.h

@@ -27,10 +27,6 @@ class SemanticsNodeBlockStack {
   // order to support lazy allocation.
   auto Push() -> void;
 
-  // Pushes a new node block.
-  // TODO: Try to remove this in favor of the lazy alloc in Push.
-  auto PushWithUnconditionalAlloc() -> SemanticsNodeBlockId;
-
   // Peeks at the top node block. This does not trigger lazy allocation, so the
   // returned node block may be invalid.
   auto Peek() -> SemanticsNodeBlockId { return stack_.back(); }