Initial migration framework (#525)

What this does:

- Sets up a `migrate_cpp` tool which currently only runs `clang-tidy`.
  - This is intended to have more transformations in the future.
- Sets up a `migrate_cpp.sh` script.
  - This copies the original woff2 code into a `carbon` directory and runs the `migrate_cpp` tool on it there.
- Adds the initial `carbon` directory of woff2
  - To be clear, this is currently only updated via `clang-tidy`.
  - More transformations should be expected in the future.
- Minor related edits. For example:
  - Adjust pre-commit to skip the `carbon` directory, because it's third-party code and shouldn't be edited in the same way.
  - Adds `@brotli_carbon` as a local repository so that we can "build" outputs.
  - Makes clang-tidy from the bootstrap toolchain accessible for BUILD dependencies, as it's then used for `migrate_cpp`.

What this does not do:

- Any actual transformation of C++ code to Carbon

Co-authored-by: Matthew Riley <mdriley@gmail.com>

migrate_cpp/migrate_cpp.py

@@ -0,0 +1,72 @@
+"""Migrates C++ code to Carbon."""
+
+__copyright__ = """
+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
+"""
+
+import argparse
+import glob
+import os
+import subprocess
+import sys
+
+_CLANG_TIDY = "../external/bootstrap_clang_toolchain/bin/clang-tidy"
+_CPP_EXTS = {".h", ".c", ".cc", ".cpp", ".cxx"}
+
+
+def _data_file(relative_path):
+    """Returns the path to a data file."""
+    return os.path.join(os.path.dirname(sys.argv[0]), relative_path)
+
+
+def _parse_args(args=None):
+    """Parses command-line arguments and flags."""
+    parser = argparse.ArgumentParser(description=__doc__)
+    parser.add_argument(
+        "dir",
+        type=str,
+        help="A directory containing C++ files to migrate to Carbon.",
+    )
+    parsed_args = parser.parse_args(args=args)
+    return parsed_args
+
+
+def _gather_files(parsed_args):
+    """Returns the list of C++ files to convert."""
+    all_files = glob.glob(
+        os.path.join(parsed_args.dir, "**/*.*"), recursive=True
+    )
+    cpp_files = [f for f in all_files if os.path.splitext(f)[1] in _CPP_EXTS]
+    if not cpp_files:
+        sys.exit(
+            "%r doesn't contain any C++ files to convert." % parsed_args.dir
+        )
+    return sorted(cpp_files)
+
+
+def _clang_tidy(parsed_args, cpp_files):
+    """Runs clang-tidy to fix C++ files in a directory."""
+    print("Running clang-tidy...")
+    clang_tidy = _data_file(_CLANG_TIDY)
+    with open(_data_file("clang_tidy.yaml")) as f:
+        config = f.read()
+    subprocess.run([clang_tidy, "--fix", "--config", config] + cpp_files)
+
+
+def _main():
+    """Main program execution."""
+    parsed_args = _parse_args()
+
+    # Validate arguments.
+    if not os.path.isdir(parsed_args.dir):
+        sys.exit("%r must point to a directory." % parsed_args.dir)
+
+    cpp_files = _gather_files(parsed_args)
+    _clang_tidy(parsed_args, cpp_files)
+    print("Done!")
+
+
+if __name__ == "__main__":
+    _main()