Finish splitting out semantics_handle.cpp (#2976)

I was running into some difficult merges in #2940, so I'm wanting to
finish splitting this last file to minimize the chance of future issues.

toolchain/semantics/semantics_handle_variable.cpp

@@ -0,0 +1,60 @@
+// 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/semantics/semantics_context.h"
+#include "toolchain/semantics/semantics_node.h"
+
+namespace Carbon {
+
+auto SemanticsHandleVariableDeclaration(SemanticsContext& context,
+                                        ParseTree::Node parse_node) -> bool {
+  // Handle the optional initializer.
+  auto expr_node_id = SemanticsNodeId::Invalid;
+  bool has_init =
+      context.parse_tree().node_kind(context.node_stack().PeekParseNode()) !=
+      ParseNodeKind::PatternBinding;
+  if (has_init) {
+    expr_node_id = context.node_stack().Pop<SemanticsNodeId>();
+    context.node_stack().PopAndDiscardSoloParseNode(
+        ParseNodeKind::VariableInitializer);
+  }
+
+  // Get the storage and add it to name lookup.
+  auto binding_id =
+      context.node_stack().Pop<SemanticsNodeId>(ParseNodeKind::PatternBinding);
+  auto binding = context.semantics_ir().GetNode(binding_id);
+  auto [name_id, storage_id] = binding.GetAsBindName();
+  context.AddNameToLookup(binding.parse_node(), name_id, storage_id);
+
+  // If there was an initializer, assign it to storage.
+  if (has_init) {
+    auto cast_value_id = context.ImplicitAsRequired(
+        parse_node, expr_node_id,
+        context.semantics_ir().GetNode(storage_id).type_id());
+    context.AddNode(SemanticsNode::Assign::Make(
+        parse_node, context.semantics_ir().GetNode(cast_value_id).type_id(),
+        storage_id, cast_value_id));
+  }
+
+  context.node_stack().PopAndDiscardSoloParseNode(
+      ParseNodeKind::VariableIntroducer);
+
+  return true;
+}
+
+auto SemanticsHandleVariableIntroducer(SemanticsContext& context,
+                                       ParseTree::Node parse_node) -> bool {
+  // No action, just a bracketing node.
+  context.node_stack().Push(parse_node);
+  return true;
+}
+
+auto SemanticsHandleVariableInitializer(SemanticsContext& context,
+                                        ParseTree::Node parse_node) -> bool {
+  // No action, just a bracketing node.
+  context.node_stack().Push(parse_node);
+  return true;
+}
+
+}  // namespace Carbon