Adds commit '79b6681aadcb53c27d1052e5f8a0e82a981dbf2f' from
https://github.com/KhronosGroup/SPIRV-Headers as
'third_party/SPIRV-Headers'

Commands:
git subtree add --prefix third_party/SPIRV-Headers
https://github.com/KhronosGroup/SPIRV-Headers
79b6681aadcb53c27d1052e5f8a0e82a981dbf2f --squash

Bug: b/123642959
Change-Id: I39d6f1aff817ec9ed527c01eb631c99d81a17a95