Pass manager: Optionally log transforms that are performed

Request from ISVs and IHVs to log transforms that are applied.

If targeting SPIR-V 1.1, then use OpModuleProcessed.
Otherwise, mimic what Glslang does to append strings.

I'd like to make this an off-by-default option on the optimizer.

Start by locating the optimizer's pass manager and how it currently handles transform execution and options. Compare the SPIR-V 1.1 OpModuleProcessed approach with Glslang's string-appending behavior; done means an off-by-default option logs performed transforms using the appropriate mechanism.

Written by the indexing model from the issue text.