Shaderc build breaks due to mismatch SPIRV-Tools w.r.t. Glslang

Shaderc bots are broken because:
- Shaderc builds are configured to use latest SPIRV-Tools
- Glslang (and its tests) assume a specific version of SPIRV-Tools
- Glslang golden test results are sensitive to SPIRV-Tools transforms
- SPIRV-Tools transforms evolve.
- So there can be a mismatch on Glslang test results in Shaderc's build bots.

The latest example is a recent improvement to block merging in SPIRV-Tools.

It's unclear how often this will occur. I'm keeping this open to document the issue, and to avoid a general panic. :-)