Split semantics into check and sem_ir directories (#3176)

Continuing along with #3070. Note this is just a file rename, with BUILD
edits; every file previously in semantics/ should show as moved (except
maybe BUILDs, which split).

toolchain/check/node_block_stack.cpp

@@ -0,0 +1,49 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/check/node_block_stack.h"
+
+#include "common/vlog.h"
+#include "llvm/ADT/STLExtras.h"
+#include "toolchain/sem_ir/node.h"
+
+namespace Carbon::Check {
+
+auto NodeBlockStack::Push(SemIR::NodeBlockId id) -> void {
+  CARBON_VLOG() << name_ << " Push " << stack_.size() << "\n";
+  CARBON_CHECK(stack_.size() < (1 << 20))
+      << "Excessive stack size: likely infinite loop";
+  stack_.push_back(id);
+}
+
+auto NodeBlockStack::PeekForAdd() -> SemIR::NodeBlockId {
+  CARBON_CHECK(!stack_.empty()) << "no current block";
+  auto& back = stack_.back();
+  if (!back.is_valid()) {
+    back = semantics_ir_->AddNodeBlock();
+    CARBON_VLOG() << name_ << " Add " << stack_.size() - 1 << ": " << back
+                  << "\n";
+  }
+  return back;
+}
+
+auto NodeBlockStack::Pop() -> SemIR::NodeBlockId {
+  CARBON_CHECK(!stack_.empty()) << "no current block";
+  auto back = stack_.pop_back_val();
+  CARBON_VLOG() << name_ << " Pop " << stack_.size() << ": " << back << "\n";
+  if (!back.is_valid()) {
+    return SemIR::NodeBlockId::Empty;
+  }
+  return back;
+}
+
+auto NodeBlockStack::PrintForStackDump(llvm::raw_ostream& output) const
+    -> void {
+  output << name_ << ":\n";
+  for (auto [i, entry] : llvm::enumerate(stack_)) {
+    output << "\t" << i << ".\t" << entry << "\n";
+  }
+}
+
+}  // namespace Carbon::Check