Move the toolchain into a top-level directory. (#567)

This should clean up our top level directory and the build patterns.

No non-mechanical edits here. Just injecting `toolchain/` and
`TOOLCHAIN_` and then running formatting tools.

toolchain/parser/parse_node_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/parser/parse_node_kind.h"
+
+#include "llvm/ADT/StringRef.h"
+
+namespace Carbon {
+
+auto ParseNodeKind::GetName() const -> llvm::StringRef {
+  static constexpr llvm::StringLiteral Names[] = {
+#define CARBON_PARSE_NODE_KIND(Name) #Name,
+#include "toolchain/parser/parse_node_kind.def"
+  };
+  return Names[static_cast<int>(kind)];
+}
+
+}  // namespace Carbon