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.

.github/workflows/linter.yml

@@ -73,9 +73,5 @@ jobs:
       - name: Configure meson
         run: mkosi sandbox -- env CC=clang CXX=clang++ meson setup build
 
-        # Make sure all generated source files are actually generated by doing a full build.
-      - name: Build systemd
-        run: mkosi sandbox -- ninja -C build
-
       - name: Run clang-tidy
         run: mkosi sandbox -- ninja -C build clang-tidy