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/handle_codeblock.cpp

@@ -0,0 +1,21 @@
+// 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/context.h"
+
+namespace Carbon::Check {
+
+auto HandleCodeBlockStart(Context& context, Parse::Node parse_node) -> bool {
+  context.node_stack().Push(parse_node);
+  context.PushScope();
+  return true;
+}
+
+auto HandleCodeBlock(Context& context, Parse::Node /*parse_node*/) -> bool {
+  context.PopScope();
+  context.node_stack().PopForSoloParseNode<Parse::NodeKind::CodeBlockStart>();
+  return true;
+}
+
+}  // namespace Carbon::Check