Implement calls in the toolchain. (#2582)

This adds tracking of call information plus basic type checking. It adds a builtin for the empty tuple, mainly so that I have the basis for a default function return type.

As an aside, it also unifies printing within SemanticsIR, fixing a missing comma after callables.

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

toolchain/semantics/semantics_node_block_stack.h

@@ -19,9 +19,10 @@ namespace Carbon {
 class SemanticsNodeBlockStack {
  public:
   explicit SemanticsNodeBlockStack(
+      llvm::StringLiteral name,
       llvm::SmallVector<llvm::SmallVector<SemanticsNodeId>>& node_blocks,
       llvm::raw_ostream* vlog_stream)
-      : node_blocks_(&node_blocks), vlog_stream_(vlog_stream) {}
+      : name_(name), node_blocks_(&node_blocks), vlog_stream_(vlog_stream) {}
 
   // Pushes a new node block. It will be invalid unless PeekForAdd is called in
   // order to support lazy allocation.
@@ -45,6 +46,9 @@ class SemanticsNodeBlockStack {
   auto size() const -> size_t { return stack_.size(); }
 
  private:
+  // A name for debugging.
+  llvm::StringLiteral name_;
+
   // The underlying node block storage on SemanticsIR. Always non-null.
   llvm::SmallVector<llvm::SmallVector<SemanticsNodeId>>* const node_blocks_;