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_pattern_binding.cpp

@@ -0,0 +1,47 @@
+// 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 SemanticsHandleAddress(SemanticsContext& context,
+                            ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "HandleAddress");
+}
+
+auto SemanticsHandleGenericPatternBinding(SemanticsContext& context,
+                                          ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "GenericPatternBinding");
+}
+
+auto SemanticsHandlePatternBinding(SemanticsContext& context,
+                                   ParseTree::Node parse_node) -> bool {
+  auto [type_node, parsed_type_id] =
+      context.node_stack().PopWithParseNode<SemanticsNodeId>();
+  auto cast_type_id = context.ExpressionAsType(type_node, parsed_type_id);
+
+  // Get the name.
+  auto [name_node, name_id] =
+      context.node_stack().PopWithParseNode<SemanticsStringId>(
+          ParseNodeKind::Name);
+
+  // Allocate storage, linked to the name for error locations.
+  auto storage_id =
+      context.AddNode(SemanticsNode::VarStorage::Make(name_node, cast_type_id));
+
+  // Bind the name to storage.
+  context.AddNodeAndPush(parse_node,
+                         SemanticsNode::BindName::Make(name_node, cast_type_id,
+                                                       name_id, storage_id));
+  return true;
+}
+
+auto SemanticsHandleTemplate(SemanticsContext& context,
+                             ParseTree::Node parse_node) -> bool {
+  return context.TODO(parse_node, "HandleTemplate");
+}
+
+}  // namespace Carbon