meson: Define our own clang-tidy target

meson's target has a few issues:
- Runs on all source files regardless if they're included in the
  build or not
- Doesn't have any dependencies on generated sources which means we
  have to do a full build first before we can run clang-tidy
- Doesn't allow us to pass any extra arguments

To work around these, let's define our own clang-tidy target instead
using llvm's run-clang-tidy script. Alongside the clang-tidy target,
let's start keeping track of all generated sources which we make the
clang-tidy target depend on. We also add a new target which will only
generate source files which is useful for setting up the source tree
for running code analysis against it.

docs/HACKING.md

@@ -331,3 +331,11 @@ compilation database used by clangd to use clang as the compiler as well:
 ```sh
 $ mkosi sandbox -- env CC=clang CXX=clang++ meson setup build
 ```
+
+Additionally, the `gensources` target can be used to make sure all generated
+sources are generated to avoid clangd complaining that these source files don't
+exist.
+
+```sh
+$ mkosi sandbox -- ninja -C build gensources
+```