* Basic support for SPIR-V 1.6
* Update SPIRV-Headers deps
* Add new environment enum for SPIR-V 1.6
* Make default environment 1.6 for most tools
* Update tests
* Disallow conditional branch with duplicate labels
* Disallow Dim=Buffer with sampled images
* Do not require the non-semantic extension after SPIR-V 1.5
This PR adds a new executable spirv-lint with a simple "Hello, world!"
program, along with its associated library and a dummy unit test.
For now, only adds to CMake and Bazel; other build systems will be added
in a future PR.
Issue: #3196
