Make add_clang_tool add a dependency on checkedc-headers.

[mattmccutchen-cci]

This ensures that building any Clang tool will automatically copy the Checked C headers to the build directory along with the Clang built-in headers (the clang-resource-headers target). The tool we currently care about is clangd, but it's good to have a generic solution in case some user tries to use some other tool in the repository that might be applicable to Checked C code (maybe clang-tidy?).

Clean up code that is now redundant:

• Remove explicit clang -> checkedc-headers dependency, since clang is defined via add_clang_tool. The clang -> clang-resource-headers dependency was already redundant, but that's an upstream Clang problem and doesn't merit introducing a diff from upstream.

• Migrate 3c from add_clang_executable to add_clang_tool, so we can remove its explicit dependencies on checkedc-headers and clang-resource-headers. A quick look at which targets in the repository use add_clang_executable versus add_clang_tool suggests that 3c should probably be using add_clang_tool anyway. In fact, it appears that clang/tools/3c/CMakeLists.txt may have been copied from clang/tools/clang-check/CMakeLists.txt back when clang-check used add_clang_executable; clang-check later switched to add_clang_tool.

There are some targets in the repository that depend on clang-resource-headers by means other than add_clang_tool. Maybe they should all depend on checkedc-headers too, but I'm unsure I can make that change safely, so I'm not doing it unless/until we find a need for it.

Testing: If I modify a checked header file and then run ninja clang, ninja 3c, or ninja clangd, the header file is copied to the build directory, as expected. Given that that works, I think it's extremely unlikely that this change to build dependencies would break any tests, so I'm not running any more tests on my side.

[dtarditi]

This looks good to me, thank you! Could you resolve the conflict, and I'll merge this.

[mattmccutchen-cci]

Thanks for the review. I resolved the conflict.

[sulekhark]

I have started the tests on ADO in order to merge this PR. Again, this is not reflected here because I had to merge master before starting the tests.

[sulekhark]

All the five Dev tests on ADO are successful.