Setup gclient and presubmit file.

This CL adds the necessary files to use gclient and the depot_tools with
the SPIRV-Tools directory. This allows doing things like `git cl format`
to format code pre-upload and `git cl presubmit -uf` to run presubmit
checks over the code.

The dependencies are all added to the DEPS file and will be
auto-downloaded. They are all pin'd to specific revisions so everyone
has the same checkout. Clang is included in the checkout so it will be
consistent over usages.

Use clang-format