Add `tools/format-cpp` script

Add `tools/format-cpp` script

The new `format-cpp` script uses `clang-format -i` to inline-format all
the C++ files except `src/cpp-common/optional/optional.hpp` and
`src/cpp-common/string_view/string_view.hpp`.

You can run it from any directory: it always runs `clang-format -i` for
the whole project.

By default, the script runs `clang-format -i`; you can override this
with the `FORMATTER` environment variable, for example:

    $ FORMATTER='clang-format-10 -i' ./tools/format-cpp

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I1049eabf8da71ce5c1d8443e1e2cdc9a73f4b4d5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/4592
Reviewed-by: Simon Marchi <simon.marchi@efficios.com>