build: implement clang-tidy using our plugin

The principle of this is that you can either externally build it with
Nix (actual implementation will be in a future commit), or it can be
built with meson if the Nix one is not passed in.

The idea I have is that dev shells don't receive the one from Nix to
avoid having to build it, but CI can use the one from Nix and save some
gratuitous rebuilds.

The design of this is that you can run `ninja -C build clang-tidy` and
it will simply correctly clang-tidy the codebase in spite of PCH
bullshit caused by the cc-wrapper.

This is a truly horrendous number of hacks in a ball, caused by bugs in
several pieces of software, and I am not even getting started.

I don't consider this to fix the clang-tidy issue filing, since we still
have a fair number of issues to fix even on the existing minimal
configuration, and I have not yet implemented it in CI. Realistically we
will need to do something like https://github.com/Ericsson/codechecker
to be able to silence warnings without physically touching the code, or
at least *diff* reports between versions.

Also, the run-clang-tidy output design is rather atrocious and must
not be inflicted upon anyone I have respect for, since it buries the
diagnostics in a pile of invocation logs. We would do really well to
integrate with the Gerrit SARIF stuff so we can dump the reports on
people in a user-friendly manner.

Related: https://git.lix.systems/lix-project/lix/issues/147

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+# Clang tidy lints for Lix
+
+This is a skeleton of a clang-tidy lints library for Lix.
+
+Currently there is one check (which is already obsolete as it has served its
+goal and is there as an example), `HasPrefixSuffixCheck`.
+
+## Running fixes/checks
+
+One file:
+
+```
+ninja -C build && clang-tidy --checks='-*,lix-*' --load=build/liblix-clang-tidy.so -p ../compile_commands.json -header-filter '\.\./src/.*\.h' --fix ../src/libcmd/installables.cc
+```
+
+Several files, in parallel:
+
+```
+ninja -C build && run-clang-tidy -checks='-*,lix-*' -load=build/liblix-clang-tidy.so -p .. -header-filter '\.\./src/.*\.h' -fix ../src | tee -a clang-tidy-result
+```
+
+## Resources
+
+* https://firefox-source-docs.mozilla.org/code-quality/static-analysis/writing-new/clang-query.html
+* https://clang.llvm.org/docs/LibASTMatchersReference.html
+* https://devblogs.microsoft.com/cppblog/exploring-clang-tooling-part-3-rewriting-code-with-clang-tidy/
+
+## Developing new checks
+
+Put something like so in `myquery.txt`:
+
+```
+set traversal     IgnoreUnlessSpelledInSource
+# ^ Ignore implicit AST nodes. May need to use AsIs depending on how you are
+# working.
+set bind-root     true
+# ^ true unless you use any .bind("foo") commands
+set print-matcher true
+enable output     dump
+match callExpr(callee(functionDecl(hasName("hasPrefix"))), optionally(hasArgument( 0, cxxConstructExpr(hasDeclaration(functionDecl(hasParameter(0, parmVarDecl(hasType(asString("const char *"))).bind("meow2"))))))))
+```
+
+Then run, e.g. `clang-query --preload hasprefix.query -p compile_commands.json src/libcmd/installables.cc`.
+
+With this you can iterate a query before writing it in C++ and suffering from
+C++.
+
+### Tips and tricks for the C++
+
+There is a function `dump()` on many things that will dump to stderr. Also
+`llvm::errs()` lets you print to stderr.
+
+When I wrote `HasPrefixSuffixCheck`, I was not really able to figure out how
+the structured replacement system was supposed to work. In principle you can
+describe the replacement with a nice DSL. Look up the Stencil system in Clang
+for details.