Start adding builtins to SemanticsIR (#2356)

This starts adding builtins with TypeType and IntegerLiteralType. Note, structurally that's all they are, and not directly accessible in any way.

Adds a type field to SemanticsNode. Now, IntegerLiteralType can be identified as having type=TypeType, and IntegerLiteral as type=IntegerLiteralType. The current iteration doesn't do anything for type propagation, because I wanted to avoid making this too big.

This also switches the Identifier IR to instead BindName, with some side-effects. I'd been trying to think how to provide a name for TypeType, and switching around how things worked seemed like a better approach. And while I think it's the right direction (e.g., alias should just be a BindName), I also realized I don't need to name TypeType: there's probably a keyword to refer to the builtin, so it shouldn't use regular name lookup.

toolchain/semantics/semantics_builtin_kind.cpp

@@ -0,0 +1,19 @@
+// 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_builtin_kind.h"
+
+#include "llvm/ADT/StringRef.h"
+
+namespace Carbon {
+
+auto SemanticsBuiltinKind::name() const -> llvm::StringRef {
+  static constexpr llvm::StringLiteral Names[] = {
+#define CARBON_SEMANTICS_BUILTIN_KIND(Name) #Name,
+#include "toolchain/semantics/semantics_builtin_kind.def"
+  };
+  return Names[static_cast<int>(kind_)];
+}
+
+}  // namespace Carbon