Run clang-tidy on headers

[jonmeow]

This patches bazel_clang_tidy handling of headers. I found an equivalent change at erenon/bazel_clang_tidy#13, but that was already rejected. Per the criticism, this will result in redundant processing of headers.

The project instead uses HeaderFilterRegex: ".*", but that results in two problems:

1. When running with -k, errors are repeated when a header is included more than once, which is common.
2. clang-tidy including errors from headers that are included from other modules (e.g., abseil-cpp); filtering correctly is difficult.

Given the trade-offs and options (including forking), I thought patching was preferable so long as it remains narrow.

[chandlerc]

LG, and makes sense about the tradeoff.

I'm gonna follow-up on the PR to try and advocate for it to get merged, but we'll see what happens.

[chandlerc]

Maybe bazel_clang_tidy to avoid any ambiguity with a patch to clang-tidy itself?

[jonmeow]

Done