Duplicate old copy of `generate_language_headers.py`

There is an old copy of the script used to generate extended instruction set headers in `utils/generate_language_headers.py` that is used when building with Bazel, this copy is lagging behind the one in the SPIRV-Headers repository (https://github.com/KhronosGroup/SPIRV-Headers/blob/main/tools/buildHeaders/bin/generate_language_headers.py).

I am not very familiar with Bazel or the scenarios in which it's used but it looks like using the script from the SPIRV-Headers repo instead of having a copy in SPIRV-Tools would prevent the two getting out of sync.