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_test.cpp

@@ -0,0 +1,25 @@
+// 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 <cstring>
+
+#include "gtest/gtest.h"
+#include "llvm/ADT/StringRef.h"
+
+namespace Carbon {
+
+namespace {
+
+// Not much to test here, so just verify that the API compiles and returns the
+// data in the `.def` file.
+#define CARBON_PARSE_NODE_KIND(Name)                   \
+  TEST(ParseNodeKindTest, Name) {                      \
+    EXPECT_EQ(#Name, ParseNodeKind::Name().GetName()); \
+  }
+#include "toolchain/parser/parse_node_kind.def"
+
+}  // namespace
+}  // namespace Carbon