Contains unfinished attempt to integrate IWYU (ref #549). To finish it different
job should be done, specifically:

- Instead of feeding IWYU with modified file a mirror source tree should be
  created with the help of CMake which will contain modified sources. This
  solves the problem with IWYU thinking that `*.generated.h` headers should be
  included in place of `*` headers.
- Build IWYU as all other third-party utilities.
- Make modified sources avoid problems with `nvim/func_attr.h` includes and
  various related tricks.

Current script may only be used for manual checks like this:

    ./scripts/check-includes.py \
        --generated-includes-dir build/include \
        --generated-includes-dir build/src/nvim/auto \
        --file src/nvim/eval/typval.c \
        -- -Isrc -Ibuild/include -Ibuild/src/nvim/auto \
           -DINCLUDE_GENERATED_DECLARATIONS

(it is also somewhat fine with `--file src/nvim/eval/typval.h`). I have no idea
why (I mean, why developer think that these lines are needed, why they are
suggested is pretty obvious: because there is typedef which mentions them before
structs are defined), but for typval.h it reports, among other things, that it
should add lines

    struct dictvar_S;
    struct listitem_S;
    struct listvar_S;
    struct listwatch_S;