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_array.cpp

@@ -0,0 +1,55 @@
+// 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"
+#include "toolchain/parser/parse_node_kind.h"
+#include "toolchain/sem_ir/node.h"
+#include "toolchain/sem_ir/node_kind.h"
+
+namespace Carbon::Check {
+
+auto HandleArrayExpressionStart(Context& /*context*/,
+                                Parse::Node /*parse_node*/) -> bool {
+  return true;
+}
+
+auto HandleArrayExpressionSemi(Context& context, Parse::Node parse_node)
+    -> bool {
+  context.node_stack().Push(parse_node);
+  return true;
+}
+
+auto HandleArrayExpression(Context& context, Parse::Node parse_node) -> bool {
+  // TODO: Handle array type with undefined bound.
+  if (context.parse_tree().node_kind(context.node_stack().PeekParseNode()) ==
+      Parse::NodeKind::ArrayExpressionSemi) {
+    context.node_stack().PopAndIgnore();
+    context.node_stack().PopAndIgnore();
+    return context.TODO(parse_node, "HandleArrayExpressionWithoutBounds");
+  }
+
+  auto bound_node_id = context.node_stack().PopExpression();
+  context.node_stack()
+      .PopAndDiscardSoloParseNode<Parse::NodeKind::ArrayExpressionSemi>();
+  auto element_type_node_id = context.node_stack().PopExpression();
+  auto bound_node = context.semantics_ir().GetNode(bound_node_id);
+  if (bound_node.kind() == SemIR::NodeKind::IntegerLiteral) {
+    auto bound_value = context.semantics_ir().GetIntegerLiteral(
+        bound_node.GetAsIntegerLiteral());
+    if (!bound_value.isNegative()) {
+      context.AddNodeAndPush(
+          parse_node,
+          SemIR::Node::ArrayType::Make(
+              parse_node, SemIR::TypeId::TypeType, bound_node_id,
+              context.ExpressionAsType(parse_node, element_type_node_id)));
+      return true;
+    }
+  }
+  CARBON_DIAGNOSTIC(InvalidArrayExpression, Error, "Invalid array expression.");
+  context.emitter().Emit(parse_node, InvalidArrayExpression);
+  context.node_stack().Push(parse_node, SemIR::NodeId::BuiltinError);
+  return true;
+}
+
+}  // namespace Carbon::Check