gen_compile_commands: prune some directories

If directories are passed to gen_compile_commands.py, os.walk() traverses
all the subdirectories to search for .cmd files, but we know some of them
are not worth traversing.

Use the 'topdown' parameter of os.walk to prune them.

Documentation about the 'topdown' option of os.walk:
  When topdown is True, the caller can modify the dirnames list
  in-place (perhaps using del or slice assignment), and walk() will
  only recurse into the subdirectories whose names remain in dirnames;
  this can be used to prune the search, impose a specific order of
  visiting, or even to inform walk() about directories the caller
  creates or renames before it resumes walk() again. Modifying
  dirnames when topdown is False has no effect on the behavior of
  the walk, because in bottom-up mode the directories in dirnames
  are generated before dirpath itself is generated.

This commit prunes four directories, .git, Documentation, include, and
tools.

The first three do not contain any C files, so skipping them makes this
script work slightly faster. My main motivation is the last one, tools/
directory.

Commit 6ca4c6d25949 ("gen_compile_commands: do not support .cmd files
under tools/ directory") stopped supporting the tools/ directory.
The current code no longer picks up .cmd files from the tools/
directory.

If you run:

  ./scripts/clang-tools/gen_compile_commands.py --log_level=INFO

then, you will see several "File ... not found" log messages.

This is expected, and I do not want to support the tools/ directory.
However, without an explicit comment "do not support tools/", somebody
might try to get it back. Clarify this.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Nathan Chancellor <nathan@kernel.org>

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