Bump external/SPIRV-Tools from 970ba4e to 6b72fe2 (#60)

Bumps [external/SPIRV-Tools](https://github.com/KhronosGroup/SPIRV-Tools) from `970ba4e` to `6b72fe2`.
- [Release notes](https://github.com/KhronosGroup/SPIRV-Tools/releases)
- [Commits](KhronosGroup/SPIRV-Tools@970ba4e...6b72fe2)

---
updated-dependencies:
- dependency-name: external/SPIRV-Tools
  dependency-type: direct:production
...

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