meson: Rework clang-tidy integration to be done via unit tests

Instead of using run-clang-tidy.py with its own scheduling, let's
just gather a list of source files ourselves and then use that to
add a unit test for each source file that runs clang-tidy on the
source file.

We also add a bit of logic to run clang-tidy on most header files
as well for extra coverage. This uncovered various header files that
were not standalone so this commit also includes fixes to make sure
the clang-tidy tests are all green.

We can also use this in a later commit to run clang-include-cleaner
on each source file in the same way.

.github/workflows/linter.yml

@@ -74,4 +74,4 @@ jobs:
         run: mkosi sandbox -- env CC=clang CXX=clang++ meson setup build
 
       - name: Run clang-tidy
-        run: mkosi sandbox -- ninja -C build clang-tidy
+        run: mkosi sandbox -- meson test -C build --suite=clang-tidy --print-errorlogs --no-stdsplit